path: root/arch/Kconfig
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authorSami Tolvanen <samitolvanen@google.com>2021-04-08 11:28:26 -0700
committerKees Cook <keescook@chromium.org>2021-04-08 16:04:20 -0700
commitcf68fffb66d60d96209446bfc4a15291dc5a5d41 (patch)
tree5d5e7905ef9b3d725bcafff40174ba98d959992a /arch/Kconfig
parente49d033bddf5b565044e2abe4241353959bc9120 (diff)
add support for Clang CFI
This change adds support for Clang’s forward-edge Control Flow Integrity (CFI) checking. With CONFIG_CFI_CLANG, the compiler injects a runtime check before each indirect function call to ensure the target is a valid function with the correct static type. This restricts possible call targets and makes it more difficult for an attacker to exploit bugs that allow the modification of stored function pointers. For more details, see: https://clang.llvm.org/docs/ControlFlowIntegrity.html Clang requires CONFIG_LTO_CLANG to be enabled with CFI to gain visibility to possible call targets. Kernel modules are supported with Clang’s cross-DSO CFI mode, which allows checking between independently compiled components. With CFI enabled, the compiler injects a __cfi_check() function into the kernel and each module for validating local call targets. For cross-module calls that cannot be validated locally, the compiler calls the global __cfi_slowpath_diag() function, which determines the target module and calls the correct __cfi_check() function. This patch includes a slowpath implementation that uses __module_address() to resolve call targets, and with CONFIG_CFI_CLANG_SHADOW enabled, a shadow map that speeds up module look-ups by ~3x. Clang implements indirect call checking using jump tables and offers two methods of generating them. With canonical jump tables, the compiler renames each address-taken function to <function>.cfi and points the original symbol to a jump table entry, which passes __cfi_check() validation. This isn’t compatible with stand-alone assembly code, which the compiler doesn’t instrument, and would result in indirect calls to assembly code to fail. Therefore, we default to using non-canonical jump tables instead, where the compiler generates a local jump table entry <function>.cfi_jt for each address-taken function, and replaces all references to the function with the address of the jump table entry. Note that because non-canonical jump table addresses are local to each component, they break cross-module function address equality. Specifically, the address of a global function will be different in each module, as it's replaced with the address of a local jump table entry. If this address is passed to a different module, it won’t match the address of the same function taken there. This may break code that relies on comparing addresses passed from other components. CFI checking can be disabled in a function with the __nocfi attribute. Additionally, CFI can be disabled for an entire compilation unit by filtering out CC_FLAGS_CFI. By default, CFI failures result in a kernel panic to stop a potential exploit. CONFIG_CFI_PERMISSIVE enables a permissive mode, where the kernel prints out a rate-limited warning instead, and allows execution to continue. This option is helpful for locating type mismatches, but should only be enabled during development. Signed-off-by: Sami Tolvanen <samitolvanen@google.com> Reviewed-by: Kees Cook <keescook@chromium.org> Tested-by: Nathan Chancellor <nathan@kernel.org> Signed-off-by: Kees Cook <keescook@chromium.org> Link: https://lore.kernel.org/r/20210408182843.1754385-2-samitolvanen@google.com
Diffstat (limited to 'arch/Kconfig')
1 files changed, 45 insertions, 0 deletions
diff --git a/arch/Kconfig b/arch/Kconfig
index ecfd3520b676..f6a85ba6cba2 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -692,6 +692,51 @@ config LTO_CLANG_THIN
If unsure, say Y.
+ bool
+ help
+ An architecture should select this option if it can support Clang's
+ Control-Flow Integrity (CFI) checking.
+config CFI_CLANG
+ bool "Use Clang's Control Flow Integrity (CFI)"
+ # Clang >= 12:
+ # - https://bugs.llvm.org/show_bug.cgi?id=46258
+ # - https://bugs.llvm.org/show_bug.cgi?id=47479
+ depends on CLANG_VERSION >= 120000
+ select KALLSYMS
+ help
+ This option enables Clang’s forward-edge Control Flow Integrity
+ (CFI) checking, where the compiler injects a runtime check to each
+ indirect function call to ensure the target is a valid function with
+ the correct static type. This restricts possible call targets and
+ makes it more difficult for an attacker to exploit bugs that allow
+ the modification of stored function pointers. More information can be
+ found from Clang's documentation:
+ https://clang.llvm.org/docs/ControlFlowIntegrity.html
+ bool "Use CFI shadow to speed up cross-module checks"
+ default y
+ depends on CFI_CLANG && MODULES
+ help
+ If you select this option, the kernel builds a fast look-up table of
+ CFI check functions in loaded modules to reduce performance overhead.
+ If unsure, say Y.
+ bool "Use CFI in permissive mode"
+ depends on CFI_CLANG
+ help
+ When selected, Control Flow Integrity (CFI) violations result in a
+ warning instead of a kernel panic. This option should only be used
+ for finding indirect call type mismatches during development.
+ If unsure, say N.

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