Saturday, March 1, 2025
Thursday, January 9, 2025
Security implications with printk
Introduction
Kernel debugging is inherently a complex task due to the intricate and low-level nature of kernel operations. Surprisingly, one of the most proficient and useful tools for tackling this challenge is theprintk function. While it may seem like a simple utility for printing messages, printk is a cornerstone of kernel debugging, offering critical insights into kernel behavior.
The printk function in the Linux kernel might appear trivial at first glance, simply serving to print messages for debugging and logging purposes. However, it is one of the most intricate and critical components of the kernel. Its complexity arises from the requirement to function reliably in all possible kernel contexts, including interrupt handlers, non-preemptive sections, and even in cases of kernel panics. This complexity has made printk a major obstacle to the integration of the preempt_rt (real-time preemption) patch into the mainline kernel, as achieving deterministic behavior and low-latency logging in real-time systems poses significant challenges.
So, kernel debugging often involves analyzing log messages to diagnose issues or understand system behavior.
Among the formats used for printing data in the kernel, %pK and %pS serve specific purposes when dealing with pointers. However, their combined usage in the same message can introduce unintended information leaks, potentially undermining Kernel Address Space Layout Randomization (KASLR) security measures.
This blog post explores the problem of combining %pK and %pS in a single message. We’ll start with an introduction to the problem, delve into how these formats work, and discuss specific scenarios, such as those involving kmemleak and module loading, where these issues can arise.
Potential Information Leak from Combining %pK and %pS
The kernel uses %pK to mask sensitive pointer addresses in logs based on the privilege level of the user reading the logs. This is particularly critical for preserving KASLR offsets, which are integral to modern system security. On the other hand, %pS resolves pointers to symbols, printing the function name and offset, or falling back to the raw address if the symbol cannot be resolved.
When %pK and %pS are used together, the masking provided by %pK can be voided if %pS prints the same address as a raw pointer. This creates a potential vector for leaking sensitive information, especially when kallsyms fails to resolve the symbol and %pS defaults to showing the raw address.Kernel Print Formats
To better understand this issue, it’s essential to look at the various print formats available in the kernel. The Documentation/core-api/printk-formats.rst provides an in-depth guide to these formats.Pointer Type Formats
The printk function offers a variety of powerful format specifiers for handling pointers, enabling developers to extract and display detailed information about kernel symbols, memory addresses, and resource ranges. Depending on the specifier, pointers passed to printk can be printed as raw addresses (%px), symbolic names with or without offsets (%pS, %ps), kernel or user memory strings (%pks, %pus), physical or DMA addresses (%pa[p], %pad), or even complex structures like resources (%pr) or ranges (%pra). Each of these formats is designed to provide flexibility and precision in debugging and introspection, often requiring integration with kernel features such as kallsyms or security mechanisms.%pS: Symbolic Representation of Function Pointers
The %pS specifier is used to print the symbolic name of a function pointer, including the offsets. For example, it outputs function_name for a given pointer. This feature relies on kallsyms, a kernel mechanism for resolving symbols, which must be enabled at build time. If kallsyms is disabled, %pS falls back to printing the raw address, as symbolic resolution is unavailable. This makes %pS an invaluable tool for debugging, providing human-readable insights into function pointers, especially in backtraces or dynamic kernel environments.
%pK: Security-Conscious Printing of Kernel Pointers
The %pK specifier addresses the security implications of exposing kernel pointers. By default, it prints masked or hashed values (e.g., 00000000) unless the kptr_restrict sysctl parameter allows unrestricted access. This behavior is essential for protecting kernel memory layout information, particularly against exploits like kernel address space layout randomization (KASLR) bypasses.
The interaction with the Linux Security Module (LSM) subsystem, such as SELinux, adds another dimension of control. When SELinux is active, additional access checks might apply, ensuring that %pK outputs are aligned with the system's security policy. For instance, even privileged users may encounter restricted pointer output if SELinux policies enforce strict controls.
Complexity Behind a Simple printk
While printk appears to be a simple logging tool, passing a pointer to it can invoke deeply integrated kernel features. Printing with %pS may involve symbol resolution and handling optional features like kallsyms, while %pK necessitates checks against security configurations and LSM policies. This intricate interplay between debugging utility and security subsystem demonstrates how printk transcends its apparent simplicity to become a critical component of kernel functionality and protection.
Real-World Scenarios: kmemleak and Module Loading
There are practical cases where the combined usage of%pK and %pS manifests. One such example is in kmemleak debugging messages. Kmemleak is a kernel memory leak detector that maintains a log of unreferenced memory allocations.
A concrete example of this issue can be seen in kmemleak debugging messages when kptr_restrict is set to 1. In this configuration, %pK effectively masks the kernel addresses to prevent leaking sensitive information. However, if %pK and %pS are used together, the masking becomes ineffective. For instance:
%08lx, %pK masks the address on the lines in the backtrace, but %pS exposes it in the fourth line if the symbol cannot be resolved. The redundancy of %pK and %pS in the same line can undermine the intended security provided by %pK.
Is this case rare or what?
The line[<0000000000000000>] 0xffffac0d743b204c appears in the log when %pS is unable to resolve an address into a symbol, falling back to printing the raw address instead. This situation is not uncommon and in this case occurs because the address corresponds to a module's initialization function that allocated the memory, is marked with the __init attribute. Functions marked as __init are automatically discarded once their execution is complete, freeing up memory. As a result, kallsyms cannot resolve the symbol since it no longer exists in the kernel's symbol table, leading to the fallback output of the raw address.
Conclusions
Combining%pK and %pS in kernel messages might seem like a harmless redundancy at first glance. However, this practice can introduce vulnerabilities by inadvertently exposing sensitive kernel information. Understanding the nuances of kernel print formats and their appropriate usage is essential for developers to maintain both system security and effective debugging capabilities.
Tuesday, October 8, 2024
Symbol suffixes compilers use to confuse developers
Ever taken a peek at the symbol table of an object or executable? Or, if you're feeling particularly adventurous, have you snooped around the Linux kernel symbol table (kallsyms, for those on a first-name basis)? If so, you've probably been baffled by the bizarre names the compiler uses to mangle your precious symbols. Well, you’re not alone. I’ve spent some quality time collecting bits and pieces of information from various corners of the internet, carefully cutting and pasting (with great skill, I might add) to create this handy page where everything is nicely grouped. If you're anything like me, you’ll appreciate having all this info in one convenient place. Enjoy the madness!
| Suffix | Description | Note |
|---|---|---|
.constprop.<n> |
Constant propagation. | Indicates that the function has been optimized using constant propagation, where constant values were propagated through the code. The |
.isra.<n> |
Interprocedural Scalar Replacement of Aggregates (ISRA). | This optimization involves breaking down aggregates (like structs or arrays) passed to functions into individual scalar values. It helps improve register usage and reduces memory accesses. The |
.clone.<n> |
Function cloning. | When the compiler creates a cloned version of a function to optimize it for specific use cases (e.g., with certain constant arguments, different calling conventions, or to assist in function inlining), it adds the .clone. |
.part.<n> |
Function partitioning. | The .part. |
.cold |
Cold code. | This suffix is added to functions or parts of functions that are considered "cold" by the compiler. Cold code refers to parts of the program that are rarely executed, such as error handling code or unlikely branches in conditionals. The compiler may optimize these functions for size rather than speed, or move them to separate sections of memory to improve cache performance for "hot" code (code that is frequently executed). |
.hot |
Hot code. | Similar to .cold, this suffix indicates "hot" code, which is frequently executed. The compiler might apply aggressive optimizations focused on improving execution speed, such as loop unrolling, function inlining, or improved branch prediction. |
.likely/.unlikely |
Likely or unlikely branches. | These suffixes indicate that the compiler has predicted whether a particular branch of code is likely or unlikely to be executed, usually based on profiling data or heuristics. The likely branch is optimized for speed, while the unlikely branch might be optimized for size or minimized in terms of performance impact. |
.lto_priv.<hash> |
Link-Time Optimization (LTO) private function. | This suffix appears during link-time optimization (LTO), where functions are optimized across translation units (source files). The .lto_priv. |
.omp_fn.<n> |
OpenMP function. | Functions generated as part of OpenMP parallelization are often given this suffix. It indicates that the function was created or modified to handle a parallel region of code as specified by OpenMP directives. The |
.split.<n> |
Split functions. | This suffix appears when a large function is split into smaller pieces, either for optimization reasons or due to certain compiler strategies (like function outlining). The |
.inline |
Inlined function. | Functions marked with this suffix have been aggressively inlined by the compiler. Sometimes, a specialized inlined version of the function is created, while the original remains intact for non-inlined calls. |
.to/.from |
Conversion functions. | These suffixes are used when functions are involved in some sort of conversion process, such as casting or transforming data structures from one form to another. .to typically refers to converting to a certain form, and .from refers to converting from a form. |
.gcda |
Profiling data generation (related to GCov). | This suffix is associated with functions that produce profiling data used by GCov (GNU's code coverage tool). These functions track execution counts and other statistics to generate code coverage information. |
.llvm.<hash> |
Local linkage promoted to External linkage | With ThinLTO, you might see mangled names having this suffix. This happens because functions inlined across units need their local references made global, causing name changes. |
Constant Propagation: Overview
Constant Propagation is an important optimization technique used by compilers to improve the performance of generated code. It involves analyzing the code to identify constant values that are known at compile-time and propagating these constants throughout the code. By substituting variables with their constant values, the compiler can simplify expressions and potentially remove unnecessary calculations, improving both the runtime performance and memory usage of the program.
How Constant Propagation Works:
Identify Constants During the compilation process, the compiler looks for variables that are assigned constant values. For example:
Here,int x = 5; int y = 2 \* x;xis a constant because it is assigned a known value5.Propagation: Once the compiler identifies a constant, it replaces the variable with its constant value wherever it appears in subsequent code. Continuing the above example:
int y = 2 * 5;Simplification: After propagation, the compiler can further simplify expressions that involve constants:
int y = 10;Dead Code Elimination: Sometimes, constant propagation leads to opportunities for other optimizations, such as dead code elimination. For instance, after propagating constants, conditional branches that always evaluate to
trueorfalsecan be simplified, allowing the compiler to remove unnecessary branches:if (5 < 10) { // This block is always executed, so the condition can be removed. }
Benefits of Constant Propagation:
- Improved Performance: Constant propagation can eliminate runtime calculations, reducing the overall number of operations in the code. This leads to faster execution times.
- Reduced Code Size: Simplifying expressions and removing redundant code can reduce the size of the compiled binary.
- Better Memory Usage: By eliminating unnecessary variables and operations, constant propagation can reduce memory consumption.
Example of Constant Propagation:
Consider the following simple C program:
void example() {
int a = 10;
int b = a + 5;
int c = b * 2;
printf("%d\n", c);
}Without constant propagation, this program might be compiled as-is, performing the operations b = a + 5 and c = b * 2 at runtime. However, with constant propagation, the compiler could optimize the program as follows:
void example() {
int c = 30;
printf("%d\n", c);
}Constant Propagation vs. Constant Folding:
- Constant Propagation focuses on replacing variables that hold constant values with those constants wherever possible in the code.
Constant Folding is another optimization technique that involves evaluating constant expressions at compile-time rather than runtime. For example:
int x = 5 + 3;Here, constant folding would replace
5 + 3with8at compile-time, eliminating the need for the addition operation at runtime.
Both techniques often work together, with constant propagation creating opportunities for constant folding and vice versa.
Types of Constant Propagation:
- Intra-Procedural Constant Propagation: This type of propagation occurs within a single function or block of code. The compiler tracks constants within the boundaries of the function or block, but it does not propagate values across different functions.
- Inter-Procedural Constant Propagation: This is a more advanced form of propagation where the compiler tracks and propagates constants across function boundaries. It requires more complex analysis but can result in better optimization, especially in large programs with function calls.
What .constprop.0 Means
- Suffix Meaning: The
.constprop.0suffix is added by the compiler (usually by GCC or Clang) to signify that the function has been subjected to constant propagation optimization. The number (0in.constprop.0) is just a version indicator and can be incremented if the function undergoes further stages of optimization. Constant Propagation at the Function Level: When a compiler identifies that certain arguments to a function are constants, it can create a specialized version of the function where those constants are hardcoded. This allows the function to be optimized more aggressively. The suffix is attached to the optimized function to distinguish it from the original unoptimized version. For example, consider the following function:
int add(int x, int y) { return x + y; }If, during optimization, the compiler finds that this function is frequently called with constant values, say
add(3, 5), it might create a specialized version of the function where the constants are propagated:int add.constprop.0() { return 8; // 3 + 5 has been precomputed }In the compiled code, this new function might be named something like
add.constprop.0to reflect that it has been optimized based on constant propagation.
When Does Constant Propagation Trigger This?
The compiler performs constant propagation across function boundaries when it can determine that certain function parameters are constant in all or some of the calls to that function. This optimization is often triggered in conjunction with inlining, constant folding, and function specialization. Here’s how it works: 1. Function Inlining: When the compiler decides to inline a function (replace the call to the function with the actual function code), it can propagate constant arguments into the inlined function. This can lead to opportunities for further simplification. If the function isn’t fully inlined for all calls, the compiler might create a specialized version with constant propagation applied for those constant cases. 2. Function Specialization: If a function is called multiple times with certain arguments that are constant, the compiler might generate a specialized version of the function for those constant values. The .constprop.0 function is such a specialized version where constant propagation and potentially other optimizations (like dead code elimination or loop unrolling) have been applied. 3. Rewriting Calls: After creating the specialized version of the function, the compiler rewrites calls to the original function with constant arguments to point to the optimized .constprop.0 version. This way, the optimized version is used where possible, but the original version remains available for cases where the arguments aren’t constant.
Benefits of .constprop.0 Functions
The creation of these specialized functions with constant propagation offers several benefits: - Performance Gains: The compiler can optimize away redundant computations and simplify the function, leading to faster execution. For example, expressions that depend on constants might be precomputed, conditional branches might be eliminated, and loops might be unrolled. - Reduced Code Size: In some cases, specialized functions with constant propagation can actually reduce the code size, as the compiler might remove code paths that are no longer needed (for example, dead branches or unnecessary variable assignments). - Better Cache Usage: Specialized versions of functions can be smaller and more cache-friendly since they focus only on the specific case where certain inputs are constant.
Example in Practice
Consider this C code:
int compute(int a, int b) {
return a * 2 + b;
}
int main() {
return compute(4, 5);
}Without optimization, the compute function would be called at runtime with the arguments 4 and 5. However, during constant propagation, the compiler detects that 4 and 5 are constants and creates a specialized version of compute:
int compute.constprop.0() {
return 13; // Precomputed: 4 * 2 + 5
}The call in main() would be replaced by a direct call to compute.constprop.0(), and no runtime multiplication or addition would be required.
Why Does the Original Function Stay?
The original, non-specialized version of the function typically stays in the binary if there are calls to it with non-constant arguments or if it cannot be fully optimized in all cases. The .constprop.0 function is just an optimized variant for cases where constants are known, so the compiler keeps both versions to handle different calling scenarios.
Possible Reasons for .inline Suffix Existence:
- Partial Inlining:
- What Happens: Sometimes, the compiler may choose to inline a function only in certain places (e.g., hot paths where performance is critical) while retaining the original non-inlined version for other calls. This can happen when the function is small enough to be inlined in performance-critical paths but also used in non-critical paths or in situations where inlining might increase code size too much.
- Result: In this case, an inlined version may be created, but the original function with an
.inlinesuffix might still be retained for non-inlined calls. This allows the compiler to balance performance and code size.
- Inlining Across Translation Units (LTO):
- What Happens: During Link-Time Optimization (LTO), functions might be inlined across different translation units (source files). However, the function might still need to be retained in its original form for other purposes (such as if it’s part of a shared library or called from another compilation unit that was not optimized in the same way).
- Result: A version of the function with the
.inlinesuffix could be preserved as an internal symbol, ensuring that the compiler can still generate callable code if needed, while simultaneously allowing aggressive inlining across units.
- Multiple Optimization Levels:
- What Happens: The compiler might generate different versions of a function to optimize for specific use cases. For instance, it could create an inlined version for certain contexts and a standalone version for others, especially if different parts of the code are compiled with different optimization flags or under different constraints (e.g., space vs. speed optimizations).
- Result: The
.inlinesuffix would then be used to distinguish the inlined version from the original, non-inlined function, even though the function is still present as a callable entity.
- Debugging and Profiling:
- What Happens: Compilers sometimes retain inlined function symbols in the binary even though the code has been inlined, for the purpose of debugging and profiling. Tools like
gdbor performance profilers may need to refer to the original function for accurate stack traces, debugging information, or code coverage data. - Result: The compiler might keep a symbol with the
.inlinesuffix so that debugging information remains available, even if the function body no longer exists in its original form.
- What Happens: Compilers sometimes retain inlined function symbols in the binary even though the code has been inlined, for the purpose of debugging and profiling. Tools like
- Function Attributes:
- What Happens: Certain function attributes or calling conventions may require that a function symbol still exists in the binary, even if the function has been inlined elsewhere. For instance, a function might be declared
inlinebut alsoweak(meaning it can be overridden) or have other attributes that necessitate keeping a symbol for linking purposes. - Result: The compiler may generate both an inlined version and retain a separate version of the function marked with
.inline, to fulfill these attributes or constraints.
- What Happens: Certain function attributes or calling conventions may require that a function symbol still exists in the binary, even if the function has been inlined elsewhere. For instance, a function might be declared
### Scenario 1: The Function Is Declared inline
When a function is explicitly declared as inline in the source code: - Expectation: The programmer indicates that they would prefer the function to be inlined to avoid the overhead of a function call. This, however, is a hint, not a guarantee. The compiler can still choose not to inline the function, especially if inlining it would increase code size excessively or if the function is too complex. - Linkage and Visibility: Typically, inline functions are defined in headers or in multiple translation units because they should be available to multiple parts of the program. If you declare a function as inline, but it has external linkage, this means the function is visible across multiple translation units, and the linker might still need to ensure that only one definition is used. As a result, the function may still need a symbol in the binary. - Compilers can generate a symbol for such inline functions, especially if they are not inlined in all cases. The symbol might be suffixed with .inline if the compiler creates a specialized version after attempting partial inlining. - Why retain a symbol?: Even though the function is marked inline, the compiler might not inline it everywhere. It might still create a regular function for some call sites while inlining others. The symbol could remain to provide an externally accessible version in case the inlining isn’t performed universally. - In Public Libraries or Interfaces: Despite being marked inline, such functions might still need Interprocedural Scalar Replacement of Aggregates (ISRA) is a compiler optimization technique aimed at improving performance by breaking down large data structures (such as arrays, structs, or classes, collectively called aggregates) into their individual scalar components (like integers or floating-point values). This allows the compiler to perform more efficient optimizations on those individual parts rather than working with the entire structure as a whole. Interprocedural means that this optimization can take place across function boundaries, not just within a single function.
Let’s explore ISRA in detail:
Key Concepts in ISRA
- Aggregate Data Structures:
- Aggregate types refer to complex data structures such as
structs,arrays, orclasses, which group together multiple individual variables into a single entity. For example, in C, a struct might look like this:
struct Point { int x; int y; };- The
Pointstructure holds two integers,xandy, as part of one entity. Passing and manipulating this entire structure at once can be inefficient, especially when only some of its fields are used in a function.
- Aggregate types refer to complex data structures such as
- Scalar Replacement:
- Scalar replacement is the process of breaking down an aggregate into its individual scalar components, such as integers, floats, or pointers. This allows the compiler to work with these smaller, more manageable parts instead of the entire structure.
- For example, the compiler could split
struct Pointinto two scalar variables,int xandint y, allowing it to perform optimizations onxandyindependently.
How ISRA Works
In the context of interprocedural optimization, ISRA looks at the data being passed between functions (i.e., across function boundaries) and determines whether the entire aggregate needs to be passed, or if the individual fields of the aggregate can be passed as independent scalars. Consider this simple example:
struct Point {
int x;
int y;
};
int computeDistance(struct Point p) {
return p.x * p.x + p.y * p.y;
}Without ISRA, the computeDistance function would take a struct Point argument by value, which means that both x and y are passed as part of the struct Point object. This may involve unnecessary memory loads, stores, and passing the entire structure on the stack.
What Happens During ISRA
ISRA optimizes this process by performing the following steps: 1. Function Argument Decomposition: - Instead of passing the entire struct Point as a single argument to computeDistance, ISRA breaks it down into its components. This means that instead of passing the structure p, the compiler will generate a version of the function that takes two int arguments, x and y: int computeDistance(int x, int y) { return x * x + y * y; } 2. Across Function Boundaries: - The key part of ISRA is that it works interprocedurally—meaning it doesn’t just happen within one function but across function calls. If a function calls computeDistance, the compiler can transform both the calling function and computeDistance so that they pass and work on the individual scalar values (x and y), instead of the entire struct Point. For example: void process() { struct Point p = {3, 4}; int d = computeDistance(p); } ISRA would convert this into: void process() { int x = 3; int y = 4; int d = computeDistance(x, y); } 3. Improved Register Utilization: - By breaking down aggregates into their scalar components, the compiler can store and manipulate those values directly in CPU registers, which are much faster than accessing memory. In the example above, the x and y values can be kept in registers instead of being stored and loaded from memory, reducing the overhead of memory access. 4. Dead Code Elimination: - If only part of the structure is used, ISRA can also help eliminate unused fields. For instance, if a function only needs p.x but not p.y, the compiler can avoid passing or loading p.y entirely. This further reduces unnecessary computation and memory access.
Example of ISRA Optimization
Before ISRA:
struct Point {
int x;
int y;
};
int computeDistance(struct Point p) {
return p.x * p.x + p.y * p.y;
}
void process() {
struct Point p = {3, 4};
int d = computeDistance(p);
}After ISRA:
int computeDistance(int x, int y) {
return x * x + y * y;
}
void process() {
int x = 3;
int y = 4;
int d = computeDistance(x, y);
}Benefits of ISRA
- Reduced Memory Traffic:
- Since scalar values (like integers and floats) can often be passed in registers, ISRA reduces the need to read from or write to memory when working with aggregate data. This leads to faster execution because memory access is generally slower than register access.
- Smaller Code Size:
- By eliminating the need to pass entire aggregates (especially if they are large), the generated code becomes smaller and more efficient, as the overhead of copying entire data structures is avoided.
- Better Cache Usage:
- ISRA reduces memory accesses, which improves cache performance. By avoiding unnecessary loads and stores of the entire structure, it minimizes cache pollution, which can result in better overall performance.
- Improved Optimizations:
- Once aggregates are replaced by scalars, the compiler can apply additional optimizations, such as constant propagation, dead code elimination, and register allocation, to individual fields, which can result in more efficient code.
Challenges and Limitations of ISRA
- Large Structures: For very large structures, ISRA may not always be beneficial because breaking them down into many scalar values can lead to high register pressure. This is especially true on architectures with limited registers, where using too many registers for scalar values can degrade performance.
- ABI Constraints: Some Application Binary Interfaces (ABI) dictate how functions should pass arguments (whether in registers or on the stack). ISRA optimizations must respect these rules, which may limit the extent to which aggregate structures can be scalarized.
- Complex Structures: ISRA is easier to apply to simple aggregates (like structs with only a few fields), but it can be more complex or impractical for deeply nested or very large structures, especially if pointers are involved.
Monday, September 30, 2024
Injecting Code on the Fly: Overcoming Challenges to produce data self stuffed binary blobs.
Sometimes you run a long-lasting process on a remote machine… For example, to compress a large file… When suddenly you have an emergency: your wife is itching to shop. At that point, you typically have a couple of options:
- Stop the job and restart it at a more convenient time.
- Keep your computer connected and go out shopping.
Surely, if you had known beforehand, you could have started the job in a screen or tmux session, but usually things didn’t go that way, and now you have to decide what to do.
If you have enough time, you can use your trusty gdb to sort this problem out. You can attach to the program, close stderr and stdout, and then create new files to replace them. You can use sigaction to disable SIGHUP, but that isn’t something you can manage when shopping is calling… You simply don’t have the time.
To address this problem, I was trying to code a simple tool to automate the gdb process.
While I was able to produce a PoC using C and Assembly, trying to have the same using pure C, I ran into a challenge that I’d like to discuss briefly and gather suggestions on, if any.
The issue is that the tool needs to inject code into the running program to replace file descriptors and disable signals. However, the code that needs to be injected might require some data.
The obvious solution would be to write a small binary in assembly that can be executed in that context, but I wanted to write it in C. The problem is: can I embed data into a function in the .text section? The assembly equivalent would be something trivial like:
jmp code data: .byte [...] code: body of the function
Doing something similar in C that is both functional and portable is far from trivial. Here’s my current solution, which still has a few issues, and I’d like to collect suggestions:
void injected_function() { volatile int a = 0; if (a) { str: asm volatile ( ".byte 0x48, 0x65, 0x6c, 0x6c, 0x6f, 0x20, 0x66, 0x72, 0x6f, 0x6d, 0x20," "0x69, 0x6e, 0x6a, 0x65, 0x63, 0x74, 0x65, 0x64, 0x20, 0x66, 0x75, 0x6e," "0x63, 0x21, 0x0a, 0x00, 0x00" ); } str_end: write(1, (void *) &&str, (&&str_end - &&str)); }
This function is supposed to be injected into the address space of a running program and write: “Hello from injected func!” However, there are a few quirks: (maybe more, but I haven’t stomped into them yet)
- In x86_64, when
-fcf-protection=fullis enabled, the asm volatile statements are considered valid jump targets, resulting in anendbr64being inserted. Solutions include skipping 2 bytes to avoid printing the opcodes of theendbr64or disabling CFI using-fcf-protection=none. I don’t like either solution, but I couldn’t find another workaround. - When compiling this for aarch64, if the message length is not a multiple of four, the resulting label of the code after the string becomes misaligned. My solution was to add a few
\x00bytes to ensure proper alignment, but I’m not satisfied with this approach.
I’m looking for a solution that is architecture-independent, but I haven’t been able to find one. Does anyone have any suggestions?
Monday, July 22, 2024
Using BTF to Build Out-of-Tree Kernel Modules with Private Struct Definitions
Introduction
OoT kernel modules often face challenges when they need access to private header struct definitions that are not available in public headers. Traditional methods to access these private headers can lead to complications and maintenance challenges. This blog post presents a PoC that demonstrates a method to write OoT kernel modules using BTF to leverage private header struct definitions. This approach aims to simplify the build process and improve maintainability.
What is BTF and Why is it in the Linux Kernel?
BPF is a technology used for network packet filtering, tracing, and monitoring within the Linux kernel. It allows users to run sandboxed programs in the kernel space, enabling powerful debugging and performance analysis capabilities. Producing BPF machine code is straightforward with a compiler that targets BPF, but writing a BPF program is more complicated due to the need to access kernel data during execution. For example, if you want to check if the IP of a given packet is your target, you need to access the structure representing the packet in your BPF program. You must navigate to the correct field by moving from the structure's address by a specific offset and interpret it correctly. This is where BTF comes into play. BTF, or BPF Type Format, is a slim and compact way to represent the structures used in the kernel, accounting for structure randomization. It provides rich type information for BPF programs, essential for accessing and manipulating kernel data structures accurately. BTF enhances the BPF ecosystem by enabling programs to understand and work with kernel data without needing explicit header files. To support BPF program development, an ecosystem has emerged, with libbpf being the key library that facilitates this. BPF programs need to be loaded into the kernel, and there is a BPF syscall for this operation. libbpf allows creating a loader program in native assembly that not only loads the program into the kernel but also links it (similar to the compiler's link process) to adapt it to the specific kernel, using BTF. Historically, Clang was the first C compiler to support the BPF target. GCC also supports the BPF target, but Clang remains the more commonly used compiler for this task. BTF focuses solely on describing data structures, which is why it is much more compact than other debugging formats like DWARF. The BTF section included in a production kernel is around 10-20MB, while DWARF info would be around 250-500MB.
Using BTF to Ease OoT Module Build and Maintenance
The PoC demonstrates how to build an OoT kernel module that requires private struct definitions by utilizing BTF. Here’s a step-by-step overview of the process:
- Search for Structure to Define: Identify the private structures and unions needed for the OoT module from the Linux headers.
- Collect All Structures and Unions: Gather all relevant structures and unions from the Linux headers.
- Extract vmlinux from Bootable Image: Extract the vmlinux file from a bootable kernel image, which contains the BTF information.
- Extract Structures from BTF: Use BTF to extract the required structures from the vmlinux file.
- Filter BTF Extracted Structures: Filter out the structures that are already declared in public headers to avoid duplication.
- Produce Header File: Generate a header file containing the necessary structures and unions.
- Build Kernel Module: Use a customized Makefile and scripts to build the kernel module with the generated header file.
The PoC includes:
- A customized Makefile that runs scripts to prepare the environment.
- Module source code that marks structures with
//BTF_INCLUDEto indicate they need to be imported. - Scripts to ensure consistency with existing structure declarations in public headers.
- Scripts to handle dependencies and recursively extract related structures without redeclaring existing ones.
Conclusion
This PoC showcases a functional solution for using BTF to build OoT kernel modules with private struct definitions. It demonstrates how BTF can be used to retrieve structure information about non-public definitions. While this PoC is not intended to promote the use of non-public structures in OoT modules, it acknowledges that sometimes this is unavoidable. Using BTF for this purpose can significantly increase the maintainability of the OoT kernel module across different kernel versions.
Thursday, June 20, 2024
Unidentified Kernel symbols: Syscall macro expansion
When navigating kernel symbols, it is not uncommon to encounter symbols that do not appear to be declared in the source code. This is often the case with symbols related to syscalls. We know that symbols are created during preprocessing (see my previous blog posts), but syscall declarations seem to be more complex. Let's look at an example:
The nice function above, after being preprocessed, spawns a few other functions:
This example is for the aarch64 architecture, but other architectures undergo the same processing.
The main function called when a syscall is invoked is __arm64_sys_test, which in turn calls __se_sys_test, and then __do_sys_test.
Please note that the user code is part of this latter function.
As we know, compilers perform complex optimizations when building user code and do not always honor the inline specifier.
This is why, when looking at symbols (for example, in kallsyms), you may or may not see do_sys_* functions.
The rationale behind this is:
Thursday, May 23, 2024
Investigate Obscure Kernel Symbols
Introduction
In the world of Linux kernel development, one often encounters intriguing anomalies that spark curiosity and investigation. My journey into exploring such peculiarities began with a previous deep dive into duplicate symbols within the Linux kernel. This exploration revealed fascinating insights into how certain symbols names, appears multiple times having different addresses. It was fun to discover that among multiple different addresses having the same name, there were also actual duplicates of the same function (name and body), even thought, the majority of those symbols having the same name were actually different objects. Building on that foundation, my current investigation delves into another set of mysterious symbols, those that appear to be aliases for given addresses in the kernel (multiple names for the same address), but whose origins are not immediately obvious. Their presence had significant consequences in my new effort. I'm currently adding a new feature to ks-nav, a nifty tool that generates diagrams from the kernel binary image. The goal is to provide kernel analysts with valuable insights into the kernel code, because who doesn't love a good kernel investigation? The tool already produces call tree diagrams and visualize subsystem interactions triggered by specific functions. My latest endeavor? To add functionality that reveals how global variables are used and shared among functions. The topic of this blog post springs from analyzing the output of this tool. Here's an image produced by investigating the global symbols shared starting from the functionhugetlb_vma_lock_alloc.
The Problem of Macro Expansion and Symbol Aliasing
Unlike the previous investigation where symbols were straightforward duplicates, the issue at hand now involves a more complex phenomenon stemming from macro expansion. The process of macro expansion in the kernel can result in multiple symbols being generated with the same name, even though, each of these are actually different variables in memory. You can have the same phenomenon originate by compiler multiple mangling of the code such as inlining, or macro expansion, but when it happens, to allow the compiler to manage these same name symbols as different, the compiler must transform these names to allow it to differentiate. In practical terms, this just means that the compiler appends numbers to the identifier name to produce a new unique identifier. A simple example can clarify this:$ cat h.c #includeint pippo(int i){ static int paperino; if (i>=0) paperino=i; return paperino; } int pluto(int i){ static int paperino; if (i>=0) paperino=i; return paperino; } int main(){ printf("paperino= %d\n", pippo(55) ); printf("paperino= %d\n", pippo(-1) ); printf("paperino= %d\n", pluto(99) ); printf("paperino= %d\n", pluto(-1) ); } $ gcc -g h.c -o h $ ./h paperino= 55 paperino= 55 paperino= 99 paperino= 99 $ nm -n h w __cxa_finalize@@GLIBC_2.2.5 w __gmon_start__ w _ITM_deregisterTMCloneTable w _ITM_registerTMCloneTable U __libc_start_main@@GLIBC_2.2.5 U printf@@GLIBC_2.2.5 0000000000001000 t _init 0000000000001060 T _start 0000000000001090 t deregister_tm_clones 00000000000010c0 t register_tm_clones 0000000000001100 t __do_global_dtors_aux 0000000000001140 t frame_dummy 0000000000001149 T pippo 000000000000116b T pluto 000000000000118d T main 0000000000001210 T __libc_csu_init 0000000000001280 T __libc_csu_fini 0000000000001288 T _fini 0000000000002000 R _IO_stdin_used 0000000000002014 r __GNU_EH_FRAME_HDR 00000000000021ac r __FRAME_END__ 0000000000003db8 d __frame_dummy_init_array_entry 0000000000003db8 d __init_array_start 0000000000003dc0 d __do_global_dtors_aux_fini_array_entry 0000000000003dc0 d __init_array_end 0000000000003dc8 d _DYNAMIC 0000000000003fb8 d _GLOBAL_OFFSET_TABLE_ 0000000000004000 D __data_start 0000000000004000 W data_start 0000000000004008 D __dso_handle 0000000000004010 B __bss_start 0000000000004010 b completed.8061 0000000000004010 D _edata 0000000000004010 D __TMC_END__ 0000000000004014 b paperino.2316 0000000000004018 b paperino.2320 0000000000004020 B _end $
This example shows, how the conflict generated by having two global variables having the same name, paperino, forced the compiler to differentiate them by appending a number. It is lesser known, but static local variables defined in functions are actually global variables. In the function namespace they do not generate any conflict, but in the compiler unit namespace they do, and this is why the compiler mangles names like that in the binary.
Back to the problem identified by the ks-nav new feature, in the diagram, there are two global data symbols that are evidently mangled by the compiler: the __key.11 and the __already_done.1
Let's start focusing on the simpler, just to familiarize with the phenomenon: the __already_done family of symbols.
The analysis evidenced it comes from pr_warn_once.
This function uses a macro to ensure that the warning message is printed only once. This mechanism ensures that each warning instance is tracked separately using a dedicated variable.
To illustrate how this works, let's track down how the pr_warn_once macro is expanded.
#define pr_warn_once(fmt, ...) \
printk_once(KERN_WARNING pr_fmt(fmt), ##__VA_ARGS__)
#define printk_once(fmt, ...) \
DO_ONCE_LITE(printk, fmt, ##__VA_ARGS__)
#define DO_ONCE_LITE(func, ...) \
DO_ONCE_LITE_IF(true, func, ##__VA_ARGS__)
#define DO_ONCE_LITE_IF(condition, func, ...) \
({ \
bool __ret_do_once = !!(condition); \
\
if (__ONCE_LITE_IF(__ret_do_once)) \
func(__VA_ARGS__); \
\
unlikely(__ret_do_once); \
})
#define __ONCE_LITE_IF(condition) \
({ \
static bool __section(".data.once") __already_done; \
bool __ret_cond = !!(condition); \
bool __ret_once = false; \
\
if (unlikely(__ret_cond && !__already_done)) { \
__already_done = true; \
__ret_once = true; \
} \
unlikely(__ret_once); \
})
The last expansion step finally provides evidences where the symbol __already_done.1 is coming from. It is easy to understand that if more than one pr_warn_once is present into the same compilation unit, the compiler ends up in having several __already_done instances actually referring different memory area, hence it is forced to change these names.
This is how __already_done.[0-9]+ symbol family is generated.
But if the compiler is so careful with names and addresses, how the aliases I mentioned at the beginning are even possible?
The Curious Case of __key Symbols
The __key family of symbols presents a different kind of anomaly.
These symbols are closely tied to the spin_lock_init function and exhibit unique behavior compared to the __already_done family.
The crux of the issue lies in how the compiler handles structures with no members in C.
In the context of the Linux kernel, when the lockdep feature is disabled (this what happen when it is enabled), the lock_class_key structure becomes an empty struct.
This means that when the compiler allocates such a variable in the data or BSS sections, it effectively allocates a zero-sized object. As a result, the next object allocated immediately afterward, ends up sharing the same address as the zero-sized object. This is the cause of the presence of these alias like symbols. They are not meant to be alias, they just happen to be such.
The __key symbols thus become aliases, purely due to the lock_class_key zero-sized nature when lockdep is disabled. This behavior is both unintended and inconsistent, as enabling lockdep causes the __key symbols to have a non-zero size, thereby
preventing them from aliasing with other symbols.
Here is an example of zero sized __key objects, compared with the same, when the lockdep is enabled:
as it appears when lockdep is disabled
$ cat System.map| grep ffffffff83534360 ffffffff83534360 b __key.11 ffffffff83534360 b __key.12 ffffffff83534360 b static_call_initialized $ readelf -Wa vmlinux |grep __key.1[12] 11513: ffffffff83534360 0 OBJECT LOCAL DEFAULT 35 __key.12 11514: ffffffff83534360 0 OBJECT LOCAL DEFAULT 35 __key.11 19420: ffffffff83541710 0 OBJECT LOCAL DEFAULT 35 __key.12 19421: ffffffff83541710 0 OBJECT LOCAL DEFAULT 35 __key.11 45259: ffffffff835690b8 0 OBJECT LOCAL DEFAULT 35 __key.11 47597: ffffffff83569b38 0 OBJECT LOCAL DEFAULT 35 __key.12 47598: ffffffff83569b38 0 OBJECT LOCAL DEFAULT 35 __key.11 51424: ffffffff8356dac0 0 OBJECT LOCAL DEFAULT 35 __key.12
readelf shows 0 sized objects, and kernel's system map shows the collision between symbols
as it appears when lockdep is enabled
$ readelf -Wa vmlinux |grep __key.1[12] 6080: ffffffff837ae610 16 OBJECT LOCAL DEFAULT 35 __key.12 6081: ffffffff837ae600 16 OBJECT LOCAL DEFAULT 35 __key.11 8402: ffffffff842624d0 16 OBJECT LOCAL DEFAULT 35 __key.11 8693: ffffffff842626b0 16 OBJECT LOCAL DEFAULT 35 __key.11 8703: ffffffff842626c0 16 OBJECT LOCAL DEFAULT 35 __key.12 8975: ffffffff84262790 16 OBJECT LOCAL DEFAULT 35 __key.12 8976: ffffffff84262780 16 OBJECT LOCAL DEFAULT 35 __key.11 10437: ffffffff84265030 16 OBJECT LOCAL DEFAULT 35 __key.11 12666: ffffffff8426ba60 16 OBJECT LOCAL DEFAULT 35 __key.12 12916: ffffffff8426bc20 16 OBJECT LOCAL DEFAULT 35 __key.12 20464: ffffffff8427b900 16 OBJECT LOCAL DEFAULT 35 __key.11 21593: ffffffff8427bb50 16 OBJECT LOCAL DEFAULT 35 __key.12 21594: ffffffff8427bb40 16 OBJECT LOCAL DEFAULT 35 __key.11 23931: ffffffff8427d240 16 OBJECT LOCAL DEFAULT 35 __key.12 23933: ffffffff8427d230 16 OBJECT LOCAL DEFAULT 35 __key.11 27527: ffffffff8428cf50 16 OBJECT LOCAL DEFAULT 35 __key.11 27902: ffffffff8428d050 16 OBJECT LOCAL DEFAULT 35 __key.12 27904: ffffffff8428d040 16 OBJECT LOCAL DEFAULT 35 __key.11 28675: ffffffff8428e1b0 16 OBJECT LOCAL DEFAULT 35 __key.11 32713: ffffffff842a0b10 16 OBJECT LOCAL DEFAULT 35 __key.12 32714: ffffffff842a0b00 16 OBJECT LOCAL DEFAULT 35 __key.11 33307: ffffffff842a2d10 16 OBJECT LOCAL DEFAULT 35 __key.11 42165: ffffffff842adb60 16 OBJECT LOCAL DEFAULT 35 __key.12 42167: ffffffff842adb50 16 OBJECT LOCAL DEFAULT 35 __key.11 44247: ffffffff842ae950 16 OBJECT LOCAL DEFAULT 35 __key.11 44865: ffffffff842aee00 16 OBJECT LOCAL DEFAULT 35 __key.12 44887: ffffffff842aedf0 16 OBJECT LOCAL DEFAULT 35 __key.11 45016: ffffffff842aeed0 16 OBJECT LOCAL DEFAULT 35 __key.12 45017: ffffffff842aeec0 16 OBJECT LOCAL DEFAULT 35 __key.11 48389: ffffffff842b0760 16 OBJECT LOCAL DEFAULT 35 __key.12 48390: ffffffff842b0750 16 OBJECT LOCAL DEFAULT 35 __key.11 49274: ffffffff842b1500 16 OBJECT LOCAL DEFAULT 35 __key.11 51779: ffffffff842b2820 16 OBJECT LOCAL DEFAULT 35 __key.12 51780: ffffffff842b2810 16 OBJECT LOCAL DEFAULT 35 __key.11 52060: ffffffff842b2cb0 16 OBJECT LOCAL DEFAULT 35 __key.12 52061: ffffffff842b2ca0 16 OBJECT LOCAL DEFAULT 35 __key.11 55853: ffffffff842b95c0 16 OBJECT LOCAL DEFAULT 35 __key.12 62007: ffffffff842cf910 16 OBJECT LOCAL DEFAULT 35 __key.12 62009: ffffffff842cf900 16 OBJECT LOCAL DEFAULT 35 __key.11 63425: ffffffff842d6580 16 OBJECT LOCAL DEFAULT 35 __key.12 63426: ffffffff842d6570 16 OBJECT LOCAL DEFAULT 35 __key.11 64498: ffffffff842d7230 16 OBJECT LOCAL DEFAULT 35 __key.12 64499: ffffffff842d7220 16 OBJECT LOCAL DEFAULT 35 __key.11 66813: ffffffff842d8710 16 OBJECT LOCAL DEFAULT 35 __key.12 66814: ffffffff842d8700 16 OBJECT LOCAL DEFAULT 35 __key.11 69350: ffffffff842d88c0 16 OBJECT LOCAL DEFAULT 35 __key.12 69351: ffffffff842d88b0 16 OBJECT LOCAL DEFAULT 35 __key.11 $ cat System.map| grep static_call_initialized ffffffff8426ba80 b static_call_initialized $ cat System.map| grep ffffffff8426ba80 ffffffff8426ba80 b static_call_initialized
as a consequence of the fact that lockdep structures are no more zero sized, the address conflict disappeared
Conclusion
The phenomena described above highlight how these lesser-known mechanisms induced a bug in the current implementation of the new ks-nav feature. It turns out ks-nav now needs a mechanism to detect zero-sized objects and skip them from evaluation. There's still work to do, but at least now I know what to blame for the hiccup. Time to teach ks-nav a new trick!