[PATCH 46/82] crypto: Refactor intentional wrap-around test

[Kees Cook]

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: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: Aditya Srivastava <yashsri421@xxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: linux-crypto@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 crypto/adiantum.c                   | 2 +-
 drivers/crypto/amcc/crypto4xx_alg.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/crypto/adiantum.c b/crypto/adiantum.c
index 60f3883b736a..c2f62ca455af 100644
--- a/crypto/adiantum.c
+++ b/crypto/adiantum.c
@@ -190,7 +190,7 @@ static inline void le128_add(le128 *r, const le128 *v1, const le128 *v2)
 
 	r->b = cpu_to_le64(x + y);
 	r->a = cpu_to_le64(le64_to_cpu(v1->a) + le64_to_cpu(v2->a) +
-			   (x + y < x));
+			   (add_would_overflow(x, y)));
 }
 
 /* Subtraction in Z/(2^{128}Z) */
diff --git a/drivers/crypto/amcc/crypto4xx_alg.c b/drivers/crypto/amcc/crypto4xx_alg.c
index e0af611a95d8..33f73234ddd9 100644
--- a/drivers/crypto/amcc/crypto4xx_alg.c
+++ b/drivers/crypto/amcc/crypto4xx_alg.c
@@ -251,7 +251,7 @@ crypto4xx_ctr_crypt(struct skcipher_request *req, bool encrypt)
 	 * the whole IV is a counter.  So fallback if the counter is going to
 	 * overlow.
 	 */
-	if (counter + nblks < counter) {
+	if (add_would_overflow(counter, nblks)) {
 		SYNC_SKCIPHER_REQUEST_ON_STACK(subreq, ctx->sw_cipher.cipher);
 		int ret;
 
-- 
2.34.1