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

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: dfbafa70 ("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>

include/linux/string.h

@@ -277,10 +277,12 @@ 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)
 
 /**
@@ -298,10 +300,11 @@ 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)
 
 /**