Re: [PATCH v7 8/8] fortify: Add Clang support

[Nick Desaulniers <ndesaulniers@google.com>]

On Tue, Feb 8, 2022 at 2:53 PM Kees Cook <keescook@chromium.org> wrote:
>
> Enable FORTIFY_SOURCE support for Clang:
>
> Use the new __pass_object_size and __overloadable attributes so that
> Clang will have appropriate visibility into argument sizes such that
> __builtin_object_size(p, 1) will behave correctly. Additional details
> available here:
>     https://github.com/llvm/llvm-project/issues/53516
>     https://github.com/ClangBuiltLinux/linux/issues/1401
>
> A bug with __builtin_constant_p() of globally defined variables was
> fixed in Clang 13 (and backported to 12.0.1), so FORTIFY support must
> depend on that version or later. Additional details here:
>     https://bugs.llvm.org/show_bug.cgi?id=41459
>     commit a52f8a59aef4 ("fortify: Explicitly disable Clang support")
>
> A bug with Clang's -mregparm=3 and -m32 makes some builtins unusable,
> so removing -ffreestanding (to gain the needed libcall optimizations
> with Clang) cannot be done. Without the libcall optimizations, Clang
> cannot provide appropriate FORTIFY coverage, so it must be disabled
> for CONFIG_X86_32. Additional details here;
>     https://github.com/llvm/llvm-project/issues/53645

Nice job on this series Kees! Hopefully we can get i386 support working next!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Cc: Miguel Ojeda <ojeda@kernel.org>
> Cc: Nick Desaulniers <ndesaulniers@google.com>
> Cc: Nathan Chancellor <nathan@kernel.org>
> Cc: George Burgess IV <gbiv@google.com>
> Cc: llvm@lists.linux.dev
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  include/linux/fortify-string.h | 40 ++++++++++++++++++++++------------
>  security/Kconfig               |  5 +++--
>  2 files changed, 29 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
> index f77cf22e2d60..295637a66c46 100644
> --- a/include/linux/fortify-string.h
> +++ b/include/linux/fortify-string.h
> @@ -4,7 +4,7 @@
>
>  #include <linux/const.h>
>
> -#define __FORTIFY_INLINE extern __always_inline __gnu_inline
> +#define __FORTIFY_INLINE extern __always_inline __gnu_inline __overloadable
>  #define __RENAME(x) __asm__(#x)
>
>  void fortify_panic(const char *name) __noreturn __cold;
> @@ -52,8 +52,17 @@ extern char *__underlying_strncpy(char *p, const char *q, __kernel_size_t size)
>  #define __underlying_strncpy   __builtin_strncpy
>  #endif
>
> +/*
> + * Clang's use of __builtin_object_size() within inlines needs hinting via
> + * __pass_object_size(). The preference is to only ever use type 1 (member
> + * size, rather than struct size), but there remain some stragglers using
> + * type 0 that will be converted in the future.
> + */
> +#define POS    __pass_object_size(1)
> +#define POS0   __pass_object_size(0)
> +
>  __FORTIFY_INLINE __diagnose_as(__builtin_strncpy, 1, 2, 3)
> -char *strncpy(char * const p, const char *q, __kernel_size_t size)
> +char *strncpy(char * const POS p, const char *q, __kernel_size_t size)
>  {
>         size_t p_size = __builtin_object_size(p, 1);
>
> @@ -65,7 +74,7 @@ char *strncpy(char * const p, const char *q, __kernel_size_t size)
>  }
>
>  __FORTIFY_INLINE __diagnose_as(__builtin_strcat, 1, 2)
> -char *strcat(char * const p, const char *q)
> +char *strcat(char * const POS p, const char *q)
>  {
>         size_t p_size = __builtin_object_size(p, 1);
>
> @@ -77,7 +86,7 @@ char *strcat(char * const p, const char *q)
>  }
>
>  extern __kernel_size_t __real_strnlen(const char *, __kernel_size_t) __RENAME(strnlen);
> -__FORTIFY_INLINE __kernel_size_t strnlen(const char * const p, __kernel_size_t maxlen)
> +__FORTIFY_INLINE __kernel_size_t strnlen(const char * const POS p, __kernel_size_t maxlen)
>  {
>         size_t p_size = __builtin_object_size(p, 1);
>         size_t p_len = __compiletime_strlen(p);
> @@ -106,7 +115,7 @@ __FORTIFY_INLINE __kernel_size_t strnlen(const char * const p, __kernel_size_t m
>         __builtin_choose_expr(__is_constexpr(__builtin_strlen(p)),      \
>                 __builtin_strlen(p), __fortify_strlen(p))
>  __FORTIFY_INLINE __diagnose_as(__builtin_strlen, 1)
> -__kernel_size_t __fortify_strlen(const char * const p)
> +__kernel_size_t __fortify_strlen(const char * const POS p)
>  {
>         __kernel_size_t ret;
>         size_t p_size = __builtin_object_size(p, 1);
> @@ -122,7 +131,7 @@ __kernel_size_t __fortify_strlen(const char * const p)
>
>  /* defined after fortified strlen to reuse it */
>  extern size_t __real_strlcpy(char *, const char *, size_t) __RENAME(strlcpy);
> -__FORTIFY_INLINE size_t strlcpy(char * const p, const char * const q, size_t size)
> +__FORTIFY_INLINE size_t strlcpy(char * const POS p, const char * const POS q, size_t size)
>  {
>         size_t p_size = __builtin_object_size(p, 1);
>         size_t q_size = __builtin_object_size(q, 1);
> @@ -149,7 +158,7 @@ __FORTIFY_INLINE size_t strlcpy(char * const p, const char * const q, size_t siz
>
>  /* defined after fortified strnlen to reuse it */
>  extern ssize_t __real_strscpy(char *, const char *, size_t) __RENAME(strscpy);
> -__FORTIFY_INLINE ssize_t strscpy(char * const p, const char * const q, size_t size)
> +__FORTIFY_INLINE ssize_t strscpy(char * const POS p, const char * const POS q, size_t size)
>  {
>         size_t len;
>         /* Use string size rather than possible enclosing struct size. */
> @@ -196,7 +205,7 @@ __FORTIFY_INLINE ssize_t strscpy(char * const p, const char * const q, size_t si
>
>  /* defined after fortified strlen and strnlen to reuse them */
>  __FORTIFY_INLINE __diagnose_as(__builtin_strncat, 1, 2, 3)
> -char *strncat(char * const p, const char * const q, __kernel_size_t count)
> +char *strncat(char * const POS p, const char * const POS q, __kernel_size_t count)
>  {
>         size_t p_len, copy_len;
>         size_t p_size = __builtin_object_size(p, 1);
> @@ -367,7 +376,7 @@ __FORTIFY_INLINE void fortify_memcpy_chk(__kernel_size_t size,
>                 memmove)
>
>  extern void *__real_memscan(void *, int, __kernel_size_t) __RENAME(memscan);
> -__FORTIFY_INLINE void *memscan(void * const p, int c, __kernel_size_t size)
> +__FORTIFY_INLINE void *memscan(void * const POS0 p, int c, __kernel_size_t size)
>  {
>         size_t p_size = __builtin_object_size(p, 0);
>
> @@ -379,7 +388,7 @@ __FORTIFY_INLINE void *memscan(void * const p, int c, __kernel_size_t size)
>  }
>
>  __FORTIFY_INLINE __diagnose_as(__builtin_memcmp, 1, 2, 3)
> -int memcmp(const void * const p, const void * const q, __kernel_size_t size)
> +int memcmp(const void * const POS0 p, const void * const POS0 q, __kernel_size_t size)
>  {
>         size_t p_size = __builtin_object_size(p, 0);
>         size_t q_size = __builtin_object_size(q, 0);
> @@ -396,7 +405,7 @@ int memcmp(const void * const p, const void * const q, __kernel_size_t size)
>  }
>
>  __FORTIFY_INLINE __diagnose_as(__builtin_memchr, 1, 2, 3)
> -void *memchr(const void * const p, int c, __kernel_size_t size)
> +void *memchr(const void * const POS0 p, int c, __kernel_size_t size)
>  {
>         size_t p_size = __builtin_object_size(p, 0);
>
> @@ -408,7 +417,7 @@ void *memchr(const void * const p, int c, __kernel_size_t size)
>  }
>
>  void *__real_memchr_inv(const void *s, int c, size_t n) __RENAME(memchr_inv);
> -__FORTIFY_INLINE void *memchr_inv(const void * const p, int c, size_t size)
> +__FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
>  {
>         size_t p_size = __builtin_object_size(p, 0);
>
> @@ -420,7 +429,7 @@ __FORTIFY_INLINE void *memchr_inv(const void * const p, int c, size_t size)
>  }
>
>  extern void *__real_kmemdup(const void *src, size_t len, gfp_t gfp) __RENAME(kmemdup);
> -__FORTIFY_INLINE void *kmemdup(const void * const p, size_t size, gfp_t gfp)
> +__FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp)
>  {
>         size_t p_size = __builtin_object_size(p, 0);
>
> @@ -433,7 +442,7 @@ __FORTIFY_INLINE void *kmemdup(const void * const p, size_t size, gfp_t gfp)
>
>  /* Defined after fortified strlen to reuse it. */
>  __FORTIFY_INLINE __diagnose_as(__builtin_strcpy, 1, 2)
> -char *strcpy(char * const p, const char * const q)
> +char *strcpy(char * const POS p, const char * const POS q)
>  {
>         size_t p_size = __builtin_object_size(p, 1);
>         size_t q_size = __builtin_object_size(q, 1);
> @@ -462,4 +471,7 @@ char *strcpy(char * const p, const char * const q)
>  #undef __underlying_strncat
>  #undef __underlying_strncpy
>
> +#undef POS
> +#undef POS0
> +
>  #endif /* _LINUX_FORTIFY_STRING_H_ */
> diff --git a/security/Kconfig b/security/Kconfig
> index 0b847f435beb..1d2d71cc1f36 100644
> --- a/security/Kconfig
> +++ b/security/Kconfig
> @@ -177,9 +177,10 @@ config HARDENED_USERCOPY_PAGESPAN
>  config FORTIFY_SOURCE
>         bool "Harden common str/mem functions against buffer overflows"
>         depends on ARCH_HAS_FORTIFY_SOURCE
> -       # https://bugs.llvm.org/show_bug.cgi?id=50322
>         # https://bugs.llvm.org/show_bug.cgi?id=41459
> -       depends on !CC_IS_CLANG
> +       depends on !CC_IS_CLANG || CLANG_VERSION >= 120001
> +       # https://github.com/llvm/llvm-project/issues/53645
> +       depends on !CC_IS_CLANG || !X86_32
>         help
>           Detect overflows of buffers in common string and memory functions
>           where the compiler can determine and validate the buffer sizes.
> --
> 2.30.2
>
>


-- 
Thanks,
~Nick Desaulniers