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string: Adjust strtomem() logic to allow for smaller sources

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[ Upstream commit 0e108725f6cc5b3be9e607f89c9fbcbb236367b7 ]

Arnd noticed we have a case where a shorter source string is being copied
into a destination byte array, but this results in a strnlen() call that
exceeds the size of the source. This is seen with -Wstringop-overread:

In file included from ../include/linux/uuid.h:11,
                 from ../include/linux/mod_devicetable.h:14,
                 from ../include/linux/cpufeature.h:12,
                 from ../arch/x86/coco/tdx/tdx.c:7:
../arch/x86/coco/tdx/tdx.c: In function 'tdx_panic.constprop':
../include/linux/string.h:284:9: error: 'strnlen' specified bound 64 exceeds source size 60 [-Werror=stringop-overread]
  284 |         memcpy_and_pad(dest, _dest_len, src, strnlen(src, _dest_len), pad); \
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../arch/x86/coco/tdx/tdx.c:124:9: note: in expansion of macro 'strtomem_pad'
  124 |         strtomem_pad(message.str, msg, '\0');
      |         ^~~~~~~~~~~~

Use the smaller of the two buffer sizes when calling strnlen(). When
src length is unknown (SIZE_MAX), it is adjusted to use dest length,
which is what the original code did.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Fixes: dfbafa70bd ("string: Introduce strtomem() and strtomem_pad()")
Tested-by: Arnd Bergmann <arnd@arndb.de>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
This commit is contained in:
Kees Cook 2023-10-18 10:53:58 -07:00 • committed by Greg Kroah-Hartman
parent 63f637309b
commit 4f834ad034
1 changed files with 5 additions and 2 deletions
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7
include/linux/string.h
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@ -276,10 +276,12 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
*/
#define strtomem_pad(dest, src, pad) do { \
const size_t _dest_len = __builtin_object_size(dest, 1); \
const size_t _src_len = __builtin_object_size(src, 1); \
\
BUILD_BUG_ON(!__builtin_constant_p(_dest_len) || \
_dest_len == (size_t)-1); \
memcpy_and_pad(dest, _dest_len, src, strnlen(src, _dest_len), pad); \
memcpy_and_pad(dest, _dest_len, src, \
strnlen(src, min(_src_len, _dest_len)), pad); \
} while (0)
/**
@ -297,10 +299,11 @@ void memcpy_and_pad(void *dest, size_t dest_len, const void *src, size_t count,
*/
#define strtomem(dest, src) do { \
const size_t _dest_len = __builtin_object_size(dest, 1); \
const size_t _src_len = __builtin_object_size(src, 1); \
\
BUILD_BUG_ON(!__builtin_constant_p(_dest_len) || \
_dest_len == (size_t)-1); \
memcpy(dest, src, min(_dest_len, strnlen(src, _dest_len))); \
memcpy(dest, src, strnlen(src, min(_src_len, _dest_len))); \
} while (0)
/**