1============================ 2Control-Flow Integrity (CFI) 3============================ 4 5This document describes the current control-flow integrity (CFI) mechanism in 6QEMU. How it can be enabled, its benefits and deficiencies, and how it affects 7new and existing code in QEMU 8 9Basics 10------ 11 12CFI is a hardening technique that focusing on guaranteeing that indirect 13function calls have not been altered by an attacker. 14The type used in QEMU is a forward-edge control-flow integrity that ensures 15function calls performed through function pointers, always call a "compatible" 16function. A compatible function is a function with the same signature of the 17function pointer declared in the source code. 18 19This type of CFI is entirely compiler-based and relies on the compiler knowing 20the signature of every function and every function pointer used in the code. 21As of now, the only compiler that provides support for CFI is Clang. 22 23CFI is best used on production binaries, to protect against unknown attack 24vectors. 25 26In case of a CFI violation (i.e. call to a non-compatible function) QEMU will 27terminate abruptly, to stop the possible attack. 28 29Building with CFI 30----------------- 31 32NOTE: CFI requires the use of link-time optimization. Therefore, when CFI is 33selected, LTO will be automatically enabled. 34 35To build with CFI, the minimum requirement is Clang 6+. If you 36are planning to also enable fuzzing, then Clang 11+ is needed (more on this 37later). 38 39Given the use of LTO, a version of AR that supports LLVM IR is required. 40The easies way of doing this is by selecting the AR provided by LLVM:: 41 42 AR=llvm-ar-9 CC=clang-9 CXX=clang++-9 /path/to/configure --enable-cfi 43 44CFI is enabled on every binary produced. 45 46If desired, an additional flag to increase the verbosity of the output in case 47of a CFI violation is offered (``--enable-debug-cfi``). 48 49Using QEMU built with CFI 50------------------------- 51 52A binary with CFI will work exactly like a standard binary. In case of a CFI 53violation, the binary will terminate with an illegal instruction signal. 54 55Incompatible code with CFI 56-------------------------- 57 58As mentioned above, CFI is entirely compiler-based and therefore relies on 59compile-time knowledge of the code. This means that, while generally supported 60for most code, some specific use pattern can break CFI compatibility, and 61create false-positives. The two main patterns that can cause issues are: 62 63* Just-in-time compiled code: since such code is created at runtime, the jump 64 to the buffer containing JIT code will fail. 65 66* Libraries loaded dynamically, e.g. with dlopen/dlsym, since the library was 67 not known at compile time. 68 69Current areas of QEMU that are not entirely compatible with CFI are: 70 711. TCG, since the idea of TCG is to pre-compile groups of instructions at 72 runtime to speed-up interpretation, quite similarly to a JIT compiler 73 742. TCI, where the interpreter has to interpret the generic *call* operation 75 763. Plugins, since a plugin is implemented as an external library 77 784. Modules, since they are implemented as an external library 79 805. Directly calling signal handlers from the QEMU source code, since the 81 signal handler may have been provided by an external library or even plugged 82 at runtime. 83 84Disabling CFI for a specific function 85------------------------------------- 86 87If you are working on function that is performing a call using an 88incompatible way, as described before, you can selectively disable CFI checks 89for such function by using the decorator ``QEMU_DISABLE_CFI`` at function 90definition, and add an explanation on why the function is not compatible 91with CFI. An example of the use of ``QEMU_DISABLE_CFI`` is provided here:: 92 93 /* 94 * Disable CFI checks. 95 * TCG creates binary blobs at runtime, with the transformed code. 96 * A TB is a blob of binary code, created at runtime and called with an 97 * indirect function call. Since such function did not exist at compile time, 98 * the CFI runtime has no way to verify its signature and would fail. 99 * TCG is not considered a security-sensitive part of QEMU so this does not 100 * affect the impact of CFI in environment with high security requirements 101 */ 102 QEMU_DISABLE_CFI 103 static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb) 104 105NOTE: CFI needs to be disabled at the **caller** function, (i.e. a compatible 106cfi function that calls a non-compatible one), since the check is performed 107when the function call is performed. 108 109CFI and fuzzing 110--------------- 111 112There is generally no advantage of using CFI and fuzzing together, because 113they target different environments (production for CFI, debug for fuzzing). 114 115CFI could be used in conjunction with fuzzing to identify a broader set of 116bugs that may not end immediately in a segmentation fault or triggering 117an assertion. However, other sanitizers such as address and ub sanitizers 118can identify such bugs in a more precise way than CFI. 119 120There is, however, an interesting use case in using CFI in conjunction with 121fuzzing, that is to make sure that CFI is not triggering any false positive 122in remote-but-possible parts of the code. 123 124CFI can be enabled with fuzzing, but with some caveats: 1251. Fuzzing relies on the linker performing function wrapping at link-time. 126The standard BFD linker does not support function wrapping when LTO is 127also enabled. The workaround is to use LLVM's lld linker. 1282. Fuzzing also relies on a custom linker script, which is only supported by 129lld with version 11+. 130 131In other words, to compile with fuzzing and CFI, clang 11+ is required, and 132lld needs to be used as a linker:: 133 134 AR=llvm-ar-11 CC=clang-11 CXX=clang++-11 /path/to/configure --enable-cfi \ 135 -enable-fuzzing --extra-ldflags="-fuse-ld=lld" 136 137and then, compile the fuzzers as usual. 138