[PATCH] string.h: Provide basic sanity checks for fallback memcpy()

[Kees Cook <kees@kernel.org>]

Instead of defining memcpy() in terms of __builtin_memcpy() deep
in arch/x86/include/asm/string_32.h, notice that it is needed up in
the general string.h, as done with other common C String APIs. This
allows us to add basic sanity checking for pathological "size"
arguments to memcpy(). Besides the run-time checking benefit, this
avoids GCC trying to be very smart about value range tracking[1] when
CONFIG_PROFILE_ALL_BRANCHES=y but FORTIFY_SOURCE=n.

Link: https://lore.kernel.org/all/202505191117.C094A90F88@keescook/ [1]
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/all/202501040747.S3LYfvYq-lkp@intel.com/
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/all/e3754f69-1dea-4542-8de0-a567a14fb95b@infradead.org/
Signed-off-by: Kees Cook <kees@kernel.org>
---
Cc: "Mickaël Salaün" <mic@digikod.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: <x86@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Shevchenko <andy@kernel.org>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: <linux-hardening@vger.kernel.org>
---
 arch/x86/include/asm/string_32.h |  6 ------
 include/linux/string.h           | 13 +++++++++++++
 2 files changed, 13 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/string_32.h b/arch/x86/include/asm/string_32.h
index 32c0d981a82a..fad9566f43a6 100644
--- a/arch/x86/include/asm/string_32.h
+++ b/arch/x86/include/asm/string_32.h
@@ -145,12 +145,6 @@ static __always_inline void *__constant_memcpy(void *to, const void *from,
 #define __HAVE_ARCH_MEMCPY
 extern void *memcpy(void *, const void *, size_t);
 
-#ifndef CONFIG_FORTIFY_SOURCE
-
-#define memcpy(t, f, n) __builtin_memcpy(t, f, n)
-
-#endif /* !CONFIG_FORTIFY_SOURCE */
-
 #define __HAVE_ARCH_MEMMOVE
 void *memmove(void *dest, const void *src, size_t n);
 
diff --git a/include/linux/string.h b/include/linux/string.h
index 01621ad0f598..a1f8fdcf8482 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -4,6 +4,7 @@
 
 #include <linux/args.h>
 #include <linux/array_size.h>
+#include <linux/bug.h>
 #include <linux/cleanup.h>	/* for DEFINE_FREE() */
 #include <linux/compiler.h>	/* for inline */
 #include <linux/types.h>	/* for size_t */
@@ -390,7 +391,19 @@ static inline const char *kbasename(const char *path)
 
 #if !defined(__NO_FORTIFY) && defined(__OPTIMIZE__) && defined(CONFIG_FORTIFY_SOURCE)
 #include <linux/fortify-string.h>
+#else
+/* Basic sanity checking even without FORTIFY_SOURCE */
+# ifndef __HAVE_ARCH_MEMCPY
+#  define memcpy(t, f, n)					\
+	do {							\
+		typeof(n) __n = (n);				\
+		/* Skip impossible sizes. */			\
+		if (!WARN_ON(__n < 0 || __n == SIZE_MAX))	\
+			__builtin_memcpy(t, f, __n);		\
+	} while (0)
+# endif
 #endif
+
 #ifndef unsafe_memcpy
 #define unsafe_memcpy(dst, src, bytes, justification)		\
 	memcpy(dst, src, bytes)
-- 
2.34.1