From: "Madhavan T. Venkataraman" <madvenka@xxxxxxxxxxxxxxxxxxx> Introduction ============ The livepatch feature requires an unwinder that can provide a reliable stack trace. General requirements for a reliable unwinder are described in this document from Mark Rutland: Documentation/livepatch/reliable-stacktrace.rst The requirements have two parts: 1. The unwinder must be enhanced with certain features. E.g., - Identifying successful termination of stack trace - Identifying unwindable and non-unwindable code - Identifying interrupts and exceptions occurring in the frame pointer prolog and epilog - Identifying features such as kretprobe and ftrace graph tracing that can modify the return address stored on the stack - Identifying corrupted/unreliable stack contents - Architecture-specific items that can render a stack trace unreliable at certain points in code 2. Validation of the frame pointer This assumes that the unwinder is based on the frame pointer (FP). The actual frame pointer that the unwinder uses cannot just be assumed to be correct. It needs to be validated somehow. This patch series is to address the following: - Identifying unwindable and non-unwindable code - Identifying interrupts and exceptions occurring in the frame pointer prolog and epilog - Validation of the frame pointer The rest are already in place AFAICT. Validation of the FP (aka FRAME_POINTER_VALIDATION) ==================== The current approach in Linux is to use objtool, a build time tool, for this purpose. When configured, objtool is invoked on every relocatable object file during kernel build. It performs static analysis of the code in each file. It walks the instructions in every function and notes the changes to the stack pointer (SP) and the frame pointer (FP). It makes sure that the changes are in accordance with the ABI rules. There are also a lot of other checks that Objtool performs. Once objtool completes successfully, the kernel can then be used for livepatch purposes. Objtool can have uses other than just FP validation. For instance, it can check control flow integrity during its analysis. Problem ======= Objtool is complex and highly architecture-dependent. There are a lot of different checks in objtool that all of the code in the kernel must pass before livepatch can be enabled. If a check fails, it must be corrected before we can proceed. Sometimes, the kernel code needs to be fixed. Sometimes, it is a compiler bug that needs to be fixed. The challenge is also to prove that all the work is complete for an architecture. As such, it presents a great challenge to enable livepatch for an architecture. A different approach ==================== I would like to propose a different approach for FP validation. I would like to be able to enable livepatch for an architecture as is. That is, without "fixing" the kernel or the compiler for it: There are three steps in this: 1. Objtool walks all the functions as usual. It computes the stack and frame pointer offsets at each instruction as usual. It generates ORC records and stores them in special sections as usual. This is simple enough to do. 2. Objtool performs validation of the offsets (see below) and checks if the frame is properly set up according to ABI rules. But the set of checks performed are a whole lot simpler than the existing Objtool checks for X86. 3. The unwinder in the kernel retrieves the ORC record for each PC in a stack trace. If there is no ORC record or if the ORC data indicates that a frame pointer has not been set up, the unwinder considers the stack frame unreliable. Otherwise, the unwinder computes a frame pointer from the ORC data. It compares the computed frame pointer with the actual frame pointer. If there is a match, the frame is reliable. If not, it isn't. A stack trace is reliable if every single frame in it is reliable. To summarize, the frame pointer validation is done dynamically instead of statically. Using this scheme, the unwinder can always know what kernel code is reliable for unwind and what is not. This is the requirement for livepatch. Instruction decoder =================== To do this, an instruction decoder needs to be implemented. I have implemented a simple, table-driven decoder for ARM64. Only a subset of the instructions needs to be fully decoded for this purpose: - Load-Store instructions - Add/Sub/Mov instructions - Branch instructions - Call instructions - Return instructions - Stack pointer authentication instruction The rest of the instructions are either dont-care from an unwind perspective or unexpected from the compiler. I have added checks for the unexpected ones to catch them if the compiler ever generates them. This decoder is simpler than a full-fledged one. But if a full-fledged one is ever implemented, my decoder can be subsumed by it. Code reorganization and reuse ============================= Stack validation scheme ----------------------- Currently, the stack validation scheme supported in Objtool is static stack validation. Static stack validation is performed in check.c. There is a lot of code in check.c that should be shared with other validation schemes such as the dynamic FP validation scheme that I am proposing. Accordingly, I have moved that code into separate files: - Code that walks instructions and decodes them - Code that manages instructions - CFI related code - Code that handles unwind hints So, all of this is shared across all architectures and validation schemes. Architecture-dependent code --------------------------- Currently, the ORC definitions and code are X86-specific. I have separated out the architecture-specific stuff from the generic stuff and placed them in appropriate files so other architectures can share. So, these are the architecture-specific parts that need to be supplied for a new architecture for my proposal. Everything else is shared. - Instruction decoder as mentioned above - ORC register definitions - ORC support functions - Unwind hint support - Invoke ORC init from kernel initialization code - Invoke ORC init from module initialization code - Add ORC_UNWIND_TABLE to kernel data in vmlinux.lds.S - Modify the unwinder to use ORC data to validate the frame pointer - Add kernel config definitions for reliable stack trace and livepatch Other than the decoder, all of this is very simple to do. Just follow the example in ARM64. For ARM64, the decoder turned out to be fairly simple. I cannot speak to other architectures. sorttable --------- At build time, the ORC tables in special sections are sorted so that the kernel does not have to spend time sorting them. The tables need to be sorted for binary search. The sorttable program works without any change in my proposal as well. FP prolog, epilog, leaf functions, generated code, etc ====================================================== If the unwinder is not able to find an ORC record for a given instruction address, it considers the code to be unreliable from an unwind perspective. This enables the unwinder to deal with: - Generated code that will not have any ORC records. If the unwinder finds an ORC record, but the record indicates that a frame pointer has not been properly set up at that instruction, then the unwinder considers that instruction unreliable from an unwind perspective. This enables the unwinder to deal with: - Low level assembly code (SYM_CODE) that is not walked by Objtool. See below. - Interrupts/exceptions in frame pointer prologs and epilogs. - Interrupts/exceptions in leaf functions that don't have a frame setup. - Compiler not setting up the frame pointer properly before calling a function. E.g., if inline assembly code occurs at the beginning of a function and it contains a call. If the unwinder finds an ORC record and the record indicates that a frame pointer has been properly set up, then it computes a frame pointer from the ORC data and compares it with the actual frame pointer. If the computed frame pointer does not match the actual one, it considers the code to be unreliable from an unwind perspective. This enables the unwinder to detect: - Cases where runtime patching of the kernel resulted in a change in the ORC for an instruction - A corrupted frame pointer Assembly functions ================== Objtool does not walk SYM_CODE functions as they are low-level functions that don't follow ABI rules or functions that manipulate register state in such a way that unwind is unreliable. For these the ORC records will show that the frame offset is 0. So, the unwinder will be able to tell that they are unreliable for unwind. As for SYM_FUNC functions, Objtool will walk them and compute ORC. However, currently, most of the SYM_FUNC functions in ARM64 do not setup a frame. So, these will look unreliable to the unwinder. While this will not impact the ability to do livepatch, I plan to submit a separate patch series to add a frame pointer prolog and epilog to many of these functions. This is to reduce the number of retries during the livepatch process. Unwind hints ============ Now, there are certain points in assembly code that we would like to unwind through reliably. Like interrupt and exception handlers. This is mainly for getting reliable stack traces in these cases and reducing the number of retries during the livepatch process. For these, unwind hints can be placed at strategic points in assembly code. Only a small number of these hints should be needed. In this work, I have defined the following unwind hints so stack traces that contain these can be done reliably: - Exception handlers - Interrupt handlers Unwind hints are collected in a special section. Objtool converts unwind hints to ORC data. The unwinder processes unwind hints to handle special cases mentioned above. Now, unwind hints are generally a problem to maintain. So, I have only defined them for the above cases. Size of the memory consumed within the kernel for this feature ============================================================== This depends on the amount of code in the kernel which, in turn, depends on the number of configs turned on. E.g., on the kernel on my arm64 system, the ORC data size for vmlinux is about 2MB. GitHub repository ================= My github repo for this version is here: https://github.com/madvenka786/linux/tree/orc_v3 Please feel free to clone and check it out. And, please let me know if you find any issues. Testing ======= - I have run all of the livepatch selftests successfully. I have written a couple of extra selftests myself which I will be posting separately. - I have a test driver to induce a NULL pointer exception to make sure that unwinding through exception handlers is reliable. - I use the test driver to create a timer to make sure that unwinding through the timer IRQ is reliable. - I call the unwinder from different places during boot to make sure that the unwinding in each of those cases is reliable. TBD === - I need to perform more rigorous testing with different scenarios. This is work in progress. Any ideas or suggestions are welcome. - I plan to add a return address check in the unwinder. The unwinder will decode the instruction at the call site for each frame and make sure that it is a valid call instruction. This is just a paranoid check to catch it if Objtool generates an incorrect ORC entry or if the FP is corrupted. --- Changelog: v3: From Mark Brown <broonie@xxxxxxxxxx> ==================================== Objtool no longer uses sub commands. I have addressed this. Objtool calls check() to perform all the validation. I have defined a check() function for the Dynamic FP Validation feature. Based on the config, the correct files will be included so that there is only one check() defined within Objtool. From Chen Zhongjin <chenzhongjin@xxxxxxxxxx> ============================================ No need to rewrite the decoder. Merge the decoder with my patch. My decoder is table based. So, it is very compact. Also, in that table, I have included checks for the instructions that alter the stack or frame pointers that the compiler should not be using. This catches all the cases where the compiler generates unexpected code. So, for now, I am keeping my decoder. But your point is a good one. May be, in the future, our decoders can be merged. Currently, both patchsets are relatively new. It is not as if either has received enough review. From Peter Zijlstra <peterz@xxxxxxxxxxxxx> ========================================================== Why can't you use the validate_branch() defined currently? Why do you want to define your own? Originally, I responded to this comment by saying that I will use validate_branch(). I studied it. It will probably take a long time for all the pieces to be in place for the current validate_branch() to work for ARM64. So, I have taken a different approach to try to shorten the time to market. In my approach, I generate the ORC data based on the actual code by walking all the instructions and following all the code paths statically. Now, a hidden code path may not be followed during my static analysis. E.g., - a retpoline is obscuring an actual branch - runtime patching can potentially change a code path In most of the cases, even if these things occur, the ORC data will not change. In the unlikely event that ORC data can be different for an instruction because of the above, the unwinder will handle that. There are two cases: 1) The SP offset or the FP offset is zero in the generated ORC. 2) The SP offset and the FP offset are both non-zero in the generated ORC. In case (1), the unwinder will return an error right away and the stack trace will be considered unreliable. So, livepatch has to retry. In case (2), the unwinder will detect the problem because the computed frame pointer will not match the actual one. So, livepatch has to retry. As long as the number of such retries is small, livepatch can easily be supported with this approach. AFAICT, the only way this will not work is if the actual frame pointer gets corrupted because of buggy kernel code and just happens to match the computed frame pointer. According to an earlier comment from Josh (IIRC), corruption of the frame pointer in the kernel is a super rare event. Even if it does occur, the frame pointer getting corrupted in a fashion that it exactly matches the computed frame pointer should be even rarer. Even if that were to happen, the unwinder checks must pass for every frame in the stack trace. This will not happen if the frame pointer is corrupted along the way. Also, if the kernel has buggy code that can corrupt the frame pointer, then all bets are off anyway. If you find any holes here, please let me know. I will work on it some more. Appreciate your feedback. I have added a number of checks to validate the CFI since version 2. They are described in: [RFC PATCH v3 13/21] objtool: arm64: Walk instructions and compute CFI for each instruction FWIW, I have also compared the CFI I am generating with DWARF information that the compiler generates. The CFIs match a 100% for Clang. In the case of gcc, the comparison fails in 1.7% of the cases. I have analyzed those cases and found the DWARF information generated by gcc is incorrect. The ORC generated by my Objtool is correct. From Miroslav Benes <mbenes@xxxxxxx> ==================================== klp_arch_set_pc() has been replaced by ftrace_instruction_pointer_set(). klp_get_ftrace_location() is not needed either. I have addressed this in version 3. From me Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx> ================================================================ - I have removed the unwind hints for FTrace and the Kretprobe trampoline. These can be added later. I have retained only the unwind hints for exceptions and interrupts. - I have enhanced the decoder to recognize the paciasp instruction. Both gcc and Clang use this to begin a frame pointer prolog. This is really useful for working out the CFI. v2: From Josh Poimboeuf <jpoimboe@xxxxxxxxxx>: ========================================== DWARF is not proven to be reliable. So, depending on it for livepatch is a problem. I have removed the DWARF part from the patch series. Instead, I have implemented the minimum ARM64 instruction decoder required for this work. I have implemented code to walk all the instructions in an object file and generate ORC data. The ORC data will be used by the unwinder to compute a frame pointer and validate the actual frame pointer with it. Unwind hints are a problem from a maintenance perspective. This is true. But there are only a few unwind hints that I have introduced in this work. Also, if an unwind hint becomes outdated, the dynamic frame pointer check will catch it so that the unwinder will know that it is unreliable. Inline ASM code can cause problems that DWARF cannot catch. Now that I walk all of the instructions, this problem is solved. In version 2, the ORC data is generated based on the actual machine code in the object file. So, the data reflects the actual code. Unreliability in any part of the code will be caught by the unwinder when it looks up the ORC data and performs a frame pointer check. Rename kernel code and data that currently contains the name dwarf to avoid confusion. This problem is solved as I have dropped DWARF altogether. Try to reuse the existing ORC data format and code as much as possible. I have reorganized the code in the following ways: - I have placed code that was in check.c in separate files so that different stack validation schemes can share the code. - I have separated architecture-specific code and structures from generic ones so that different architectures can share common stuff. - I am using the ORC structure as it is currently defined. The only cosmetic change I have done is to rename the fields bp_* to fp_* (FP for frame pointer). - I completely reuse the ORC definitions and code. E.g., in the kernel the ORC lookup code is shared across architectures. Objtool contains other features which other architectures are looking into. So, should we just implement static stack validation for other architectures or use dynamic FP validation just for the livepatch feature? For one thing, it will take a long time before the static validation scheme can even be proved to be complete on ARM64. Livepatching is an immediate need for security fixes. Also, since I am using the traditional approach in v2 of walking the instructions, computing CFI, generating ORC, etc, my current approach can be combined with the traditional approach. Dynamic FP validation can be offered either as an alternative to static stack validation or as something that can be combined with static stack validation to make the feature even more robust. Objtool can always have bugs and there can be bad ORC data. From Peter Zijlstra <peterz@xxxxxxxxxxxxx>: =========================================== Please use/extend ORC. Done. Please see my description above. Why deviate from the traditional approach of static stack validation? I have given the answer above. Mandating DWARF sucks. Compile times are so much worse with DWARVES. I have removed DWARF from the work. I use the traditional approach of decoding the instructions and computing the same data as DWARF but in ORC format. DWARF does not cover assembly code. In v2, I walk all of the functions including assembly functions. SYM_CODE functions are not walked by Objtool anyway. So, I don't do that. But I walk all the SYM_FUNC functions. Currently, only a few SYM_FUNC functions in ARM64 have a proper FP prolog and epilog. So, I plan to submit a separate patch series to add an FP prolog and epilog for other SYM_FUNC functions. But this is only to reduce retries during the livepatch process. It is not absolutely required for livepatch to work. But I plan to address this separately. Compilers don't consider DWARF generation to be a correctness issue. I totally agree. I have myself found 4 bugs that I have had to compensate for. So, I have dropped DWARF. From Chen Zhongjin <chenzhongjin@xxxxxxxxxx>: ============================================= One cannot depend on compilers to generate correct DWARF info. Agreed. I have dropped DWARF. DWARF does not cover assembly. So, what if too many assembly functions exist so that the livepatch process can encounter too many retries? DWARF has been dropped. The code in version 2 walks all the functions including assembly functions. There is a corner case where an interrupt or an exception can happen in FP prologs/epilogs. The stack trace would be unreliable. Yes. This will be caught by the reliable unwinder in the kernel in my scheme when it retrieves the ORC data and validates the actual frame pointer. The validation will fail and the stack trace will be considered unreliable. v1: - Introduced the livepatch feature based on DWARF Call Frame Information generated by the compilers. Previous versions and discussion ================================ v2: https://lore.kernel.org/linux-arm-kernel/20220524001637.1707472-1-madvenka@xxxxxxxxxxxxxxxxxxx/T/#t v1: https://lore.kernel.org/linux-arm-kernel/20220407202518.19780-1-madvenka@xxxxxxxxxxxxxxxxxxx/T/#t Madhavan T. Venkataraman (22): objtool: Reorganize CFI code objtool: Reorganize instruction-related code objtool: Move decode_instructions() to a separate file objtool: Reorganize Unwind hint code objtool: Reorganize ORC types objtool: Reorganize ORC code objtool: Reorganize ORC kernel code objtool: Introduce STATIC_CHECK objtool: arm64: Add basic definitions and compile objtool: arm64: Implement decoder for Dynamic FP validation objtool: arm64: Invoke the decoder objtool: arm64: Compute destinations for call and jump instructions objtool: arm64: Walk instructions and compute CFI for each instruction objtool: arm64: Generate ORC data from CFI for object files objtool: arm64: Add unwind hint support arm64: Add unwind hints to exception handlers arm64: Add kernel and module support for ORC arm64: Build the kernel with ORC information arm64: unwinder: Add a reliability check in the unwinder based on ORC arm64: Define HAVE_DYNAMIC_FTRACE_WITH_ARGS arm64: Define TIF_PATCH_PENDING for livepatch arm64: Enable livepatch for ARM64 arch/arm64/Kconfig | 5 + arch/arm64/Kconfig.debug | 33 + arch/arm64/include/asm/ftrace.h | 20 + arch/arm64/include/asm/module.h | 12 +- arch/arm64/include/asm/orc_types.h | 35 ++ arch/arm64/include/asm/stacktrace/common.h | 15 + arch/arm64/include/asm/thread_info.h | 4 +- arch/arm64/include/asm/unwind_hints.h | 104 ++++ arch/arm64/kernel/entry.S | 3 + arch/arm64/kernel/module.c | 13 +- arch/arm64/kernel/setup.c | 2 + arch/arm64/kernel/signal.c | 4 + arch/arm64/kernel/stacktrace.c | 167 ++++- arch/arm64/kernel/vmlinux.lds.S | 3 + arch/x86/include/asm/orc_types.h | 37 +- arch/x86/include/asm/unwind.h | 5 - arch/x86/include/asm/unwind_hints.h | 83 +++ arch/x86/kernel/module.c | 7 +- arch/x86/kernel/unwind_orc.c | 258 +------- arch/x86/kernel/vmlinux.lds.S | 2 +- .../asm => include/asm-generic}/orc_lookup.h | 42 ++ include/linux/objtool.h | 72 +-- include/linux/orc_entry.h | 39 ++ kernel/Makefile | 2 + kernel/orc_lookup.c | 261 ++++++++ scripts/Makefile | 4 +- scripts/Makefile.lib | 9 + tools/arch/arm64/include/asm/orc_types.h | 35 ++ tools/arch/arm64/include/asm/unwind_hints.h | 104 ++++ tools/arch/x86/include/asm/orc_types.h | 37 +- tools/arch/x86/include/asm/unwind_hints.h | 157 +++++ tools/include/linux/objtool.h | 72 +-- tools/include/linux/orc_entry.h | 39 ++ tools/objtool/Build | 9 +- tools/objtool/Makefile | 8 +- tools/objtool/arch/arm64/Build | 2 + tools/objtool/arch/arm64/decode.c | 573 ++++++++++++++++++ .../arch/arm64/include/arch/cfi_regs.h | 13 + tools/objtool/arch/arm64/include/arch/elf.h | 9 + .../arch/arm64/include/arch/endianness.h | 9 + tools/objtool/arch/arm64/orc.c | 90 +++ tools/objtool/arch/x86/Build | 1 + tools/objtool/arch/x86/include/arch/elf.h | 1 + tools/objtool/arch/x86/orc.c | 150 +++++ tools/objtool/cfi.c | 108 ++++ tools/objtool/check.c | 492 +-------------- tools/objtool/dcheck.c | 360 +++++++++++ tools/objtool/decode.c | 107 ++++ tools/objtool/include/objtool/arch.h | 2 + tools/objtool/include/objtool/cfi.h | 12 + tools/objtool/include/objtool/check.h | 77 +-- tools/objtool/include/objtool/endianness.h | 1 + tools/objtool/include/objtool/insn.h | 130 ++++ tools/objtool/include/objtool/objtool.h | 1 + tools/objtool/include/objtool/orc.h | 18 + tools/objtool/insn.c | 215 +++++++ tools/objtool/orc_dump.c | 63 +- tools/objtool/orc_gen.c | 89 +-- tools/objtool/sync-check.sh | 10 + tools/objtool/unwind_hints.c | 110 ++++ 60 files changed, 3176 insertions(+), 1169 deletions(-) create mode 100644 arch/arm64/include/asm/orc_types.h create mode 100644 arch/arm64/include/asm/unwind_hints.h rename {arch/x86/include/asm => include/asm-generic}/orc_lookup.h (51%) create mode 100644 include/linux/orc_entry.h create mode 100644 kernel/orc_lookup.c create mode 100644 tools/arch/arm64/include/asm/orc_types.h create mode 100644 tools/arch/arm64/include/asm/unwind_hints.h create mode 100644 tools/arch/x86/include/asm/unwind_hints.h create mode 100644 tools/include/linux/orc_entry.h create mode 100644 tools/objtool/arch/arm64/Build create mode 100644 tools/objtool/arch/arm64/decode.c create mode 100644 tools/objtool/arch/arm64/include/arch/cfi_regs.h create mode 100644 tools/objtool/arch/arm64/include/arch/elf.h create mode 100644 tools/objtool/arch/arm64/include/arch/endianness.h create mode 100644 tools/objtool/arch/arm64/orc.c create mode 100644 tools/objtool/arch/x86/orc.c create mode 100644 tools/objtool/cfi.c create mode 100644 tools/objtool/dcheck.c create mode 100644 tools/objtool/decode.c create mode 100644 tools/objtool/include/objtool/insn.h create mode 100644 tools/objtool/include/objtool/orc.h create mode 100644 tools/objtool/insn.c create mode 100644 tools/objtool/unwind_hints.c base-commit: 830b3c68c1fb1e9176028d02ef86f3cf76aa2476 -- 2.25.1
