Messages in this thread Patch in this message | | | Date | Wed, 13 Oct 2021 11:16:47 -0700 | Subject | [PATCH v5 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB | From | Sami Tolvanen <> |
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This change introduces the DEFINE_CFI_IMMEDIATE_RETURN_STUB macro, which defines a stub function that immediately returns and when defined in the core kernel, always passes indirect call checking with CONFIG_CFI_CLANG. Note that this macro should only be used when a stub cannot be called using the correct function type.
Signed-off-by: Sami Tolvanen <samitolvanen@google.com> Tested-by: Nick Desaulniers <ndesaulniers@google.com> Tested-by: Sedat Dilek <sedat.dilek@gmail.com> --- include/asm-generic/vmlinux.lds.h | 11 +++++++++++ include/linux/cfi.h | 13 +++++++++++++ kernel/cfi.c | 24 +++++++++++++++++++++++- 3 files changed, 47 insertions(+), 1 deletion(-)
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h index f2984af2b85b..5b77284f7221 100644 --- a/include/asm-generic/vmlinux.lds.h +++ b/include/asm-generic/vmlinux.lds.h @@ -407,6 +407,16 @@ KEEP(*(.static_call_tramp_key)) \ __stop_static_call_tramp_key = .; +#ifdef CONFIG_CFI_CLANG +#define CFI_EXCLUDED_DATA \ + . = ALIGN(8); \ + __start_cfi_excluded = .; \ + KEEP(*(.cfi_excluded_stubs)) \ + __stop_cfi_excluded = .; +#else +#define CFI_EXCLUDED_DATA +#endif + /* * Allow architectures to handle ro_after_init data on their * own by defining an empty RO_AFTER_INIT_DATA. @@ -430,6 +440,7 @@ __start_rodata = .; \ *(.rodata) *(.rodata.*) \ SCHED_DATA \ + CFI_EXCLUDED_DATA \ RO_AFTER_INIT_DATA /* Read only after init */ \ . = ALIGN(8); \ __start___tracepoints_ptrs = .; \ diff --git a/include/linux/cfi.h b/include/linux/cfi.h index 879744aaa6e0..19f74af8eac2 100644 --- a/include/linux/cfi.h +++ b/include/linux/cfi.h @@ -20,6 +20,17 @@ extern void __cfi_check(uint64_t id, void *ptr, void *diag); #define __CFI_ADDRESSABLE(fn, __attr) \ const void *__cfi_jt_ ## fn __visible __attr = (void *)&fn +/* + * Defines a stub function that returns immediately, and when defined and + * referenced in the core kernel, always passes CFI checking. This should + * be used only for stubs that cannot be called using the correct function + * pointer type, which should be rare. + */ +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \ + void fn(void) { return; } \ + const void *__cfi_excl_ ## fn __visible \ + __section(".cfi_excluded_stubs") = (void *)&fn + #ifdef CONFIG_CFI_CLANG_SHADOW extern void cfi_module_add(struct module *mod, unsigned long base_addr); @@ -35,6 +46,8 @@ static inline void cfi_module_remove(struct module *mod, unsigned long base_addr #else /* !CONFIG_CFI_CLANG */ #define __CFI_ADDRESSABLE(fn, __attr) +#define DEFINE_CFI_IMMEDIATE_RETURN_STUB(fn) \ + void fn(void) { return; } #endif /* CONFIG_CFI_CLANG */ diff --git a/kernel/cfi.c b/kernel/cfi.c index 9594cfd1cf2c..8d931089141b 100644 --- a/kernel/cfi.c +++ b/kernel/cfi.c @@ -278,12 +278,34 @@ static inline cfi_check_fn find_module_check_fn(unsigned long ptr) return fn; } +extern unsigned long __start_cfi_excluded[]; +extern unsigned long __stop_cfi_excluded[]; + +static inline bool is_cfi_excluded(unsigned long ptr) +{ + unsigned long *p = __start_cfi_excluded; + + for ( ; p < __stop_cfi_excluded; ++p) + if (*p == ptr) + return true; + + return false; +} + +static void __cfi_pass(uint64_t id, void *ptr, void *diag) +{ +} + static inline cfi_check_fn find_check_fn(unsigned long ptr) { cfi_check_fn fn = NULL; - if (is_kernel_text(ptr)) + if (is_kernel_text(ptr)) { + if (unlikely(is_cfi_excluded(ptr))) + return __cfi_pass; + return __cfi_check; + } /* * Indirect call checks can happen when RCU is not watching. Both -- 2.33.0.1079.g6e70778dc9-goog
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