[PATCH] x86: string_32.h: Provide basic sanity checks for fallback memcpy()

[Kees Cook <kees@kernel.org>]

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: Randy Dunlap <rdunlap@infradead.org>
Closes: https://lore.kernel.org/all/e3754f69-1dea-4542-8de0-a567a14fb95b@infradead.org/
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Kees Cook <kees@kernel.org>
---
 v2: isolate this specifically to 32-bit x86 -- doing this generally is much more work
 v1: https://lore.kernel.org/lkml/20250520163320.work.924-kees@kernel.org/
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
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: Randy Dunlap <rdunlap@infradead.org>
---
 arch/x86/include/asm/string_32.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/string_32.h b/arch/x86/include/asm/string_32.h
index 32c0d981a82a..6e8d100d1301 100644
--- a/arch/x86/include/asm/string_32.h
+++ b/arch/x86/include/asm/string_32.h
@@ -147,7 +147,14 @@ extern void *memcpy(void *, const void *, size_t);
 
 #ifndef CONFIG_FORTIFY_SOURCE
 
-#define memcpy(t, f, n) __builtin_memcpy(t, f, n)
+#define memcpy(t, f, n)					\
+	({						\
+		typeof(n) __n = (n);			\
+		/* Skip impossible sizes. */		\
+		if (!(__n < 0 || __n == SIZE_MAX))	\
+			__builtin_memcpy(t, f, __n);	\
+		(t);					\
+	})
 
 #endif /* !CONFIG_FORTIFY_SOURCE */
 
-- 
2.34.1