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fortify: Add Clang support
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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:
    llvm/llvm-project#53516
    ClangBuiltLinux#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 a52f8a5 ("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;
    llvm/llvm-project#53645

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>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lore.kernel.org/r/20220208225350.1331628-9-keescook@chromium.org
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@kees
kees committed Feb 9, 2022
1 parent 9f51c7b commit e9ebe9d
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Showing 2 changed files with 29 additions and 16 deletions.
40 changes: 26 additions & 14 deletions include/linux/fortify-string.h
View file
Expand Up @@ -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;
Expand Down Expand Up @@ -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);

Expand All @@ -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);

Expand All @@ -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);
Expand Down Expand Up @@ -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);
Expand All @@ -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);
Expand All @@ -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. */
Expand Down Expand Up @@ -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);
Expand Down Expand Up @@ -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);

Expand All @@ -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);
Expand All @@ -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);

Expand All @@ -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);

Expand All @@ -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);

Expand All @@ -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);
Expand Down Expand Up @@ -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_ */
5 changes: 3 additions & 2 deletions security/Kconfig
View file
Expand Up @@ -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.
Expand Down

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