Re: [PATCH v4 04/15] cfi: Add DEFINE_CFI_IMMEDIATE_RETURN_STUB

Nick Desaulniers
2021-09-30T18:50:30Z

On Thu, Sep 30, 2021 at 11:05 AM Sami Tolvanen <samitolvanen@google.com> wrote:
>
> 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.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

Seems like the only use is in patch 5/15. Probably could be squashed...

>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.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
> --
> 2.33.0.800.g4c38ced690-goog
>

--
Thanks,
~Nick Desaulniers