[PATCH 56/82] usercopy: Refactor intentional wrap-around test

[Kees Cook <keescook@chromium.org>]

In an effort to separate intentional arithmetic wrap-around from
unexpected wrap-around, we need to refactor places that depend on this
kind of math. One of the most common code patterns of this is:

	VAR + value < VAR

Notably, this is considered "undefined behavior" for signed and pointer
types, which the kernel works around by using the -fno-strict-overflow
option in the build[1] (which used to just be -fwrapv). Regardless, we
want to get the kernel source to the position where we can meaningfully
instrument arithmetic wrap-around conditions and catch them when they
are unexpected, regardless of whether they are signed[2], unsigned[3],
or pointer[4] types.

Refactor open-coded wrap-around addition test to use add_would_overflow().
This paves the way to enabling the wrap-around sanitizers in the future.

Link: https://git.kernel.org/linus/68df3755e383e6fecf2354a67b08f92f18536594 [1]
Link: https://github.com/KSPP/linux/issues/26 [2]
Link: https://github.com/KSPP/linux/issues/27 [3]
Link: https://github.com/KSPP/linux/issues/344 [4]
Cc: Kees Cook <keescook@chromium.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-hardening@vger.kernel.org
Cc: linux-mm@kvack.org
Cc: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 mm/usercopy.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mm/usercopy.c b/mm/usercopy.c
index 83c164aba6e0..5141c4402903 100644
--- a/mm/usercopy.c
+++ b/mm/usercopy.c
@@ -151,7 +151,7 @@ static inline void check_bogus_address(const unsigned long ptr, unsigned long n,
 				       bool to_user)
 {
 	/* Reject if object wraps past end of memory. */
-	if (ptr + (n - 1) < ptr)
+	if (add_would_overflow(ptr, (n - 1)))
 		usercopy_abort("wrapped address", NULL, to_user, 0, ptr + n);
 
 	/* Reject if NULL or ZERO-allocation. */
-- 
2.34.1