[PATCH v2] string: Add __realloc_size hint to kmemdup()

[Kees Cook]

Add __realloc_size() hint to kmemdup() so the compiler can reason about
the length of the returned buffer. (These must not use __alloc_size,
since those include __malloc which says the contents aren't defined[1]).

[1] https://lore.kernel.org/linux-hardening/d199c2af-06af-8a50-a6a1-00eefa0b67b4@xxxxxxxxx/

Cc: Rasmus Villemoes <rasmus.villemoes@xxxxxxxxx>
Cc: Guenter Roeck <linux@xxxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
Cc: Paolo Abeni <pabeni@xxxxxxxxxx>
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
v2: use __realloc_size() instead of __alloc_size()
v1: https://lore.kernel.org/linux-hardening/20221018082232.never.213-kees@xxxxxxxxxx/
---
 include/linux/fortify-string.h | 3 ++-
 include/linux/string.h         | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/fortify-string.h b/include/linux/fortify-string.h
index e5b39b1cc2fc..49782f63f015 100644
--- a/include/linux/fortify-string.h
+++ b/include/linux/fortify-string.h
@@ -659,7 +659,8 @@ __FORTIFY_INLINE void *memchr_inv(const void * const POS0 p, int c, size_t size)
 	return __real_memchr_inv(p, c, size);
 }
 
-extern void *__real_kmemdup(const void *src, size_t len, gfp_t gfp) __RENAME(kmemdup);
+extern void *__real_kmemdup(const void *src, size_t len, gfp_t gfp) __RENAME(kmemdup)
+								    __realloc_size(2);
 __FORTIFY_INLINE void *kmemdup(const void * const POS0 p, size_t size, gfp_t gfp)
 {
 	size_t p_size = __struct_size(p);
diff --git a/include/linux/string.h b/include/linux/string.h
index cf7607b32102..db28802ab0a6 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -176,7 +176,7 @@ extern void kfree_const(const void *x);
 extern char *kstrdup(const char *s, gfp_t gfp) __malloc;
 extern const char *kstrdup_const(const char *s, gfp_t gfp);
 extern char *kstrndup(const char *s, size_t len, gfp_t gfp);
-extern void *kmemdup(const void *src, size_t len, gfp_t gfp);
+extern void *kmemdup(const void *src, size_t len, gfp_t gfp) __realloc_size(2);
 extern char *kmemdup_nul(const char *s, size_t len, gfp_t gfp);
 
 extern char **argv_split(gfp_t gfp, const char *str, int *argcp);
-- 
2.34.1