crypto: arm64/aes-ctr - fix NULL dereference in tail processing

The AES-CTR glue code avoids calling into the blkcipher API for the
tail portion of the walk, by comparing the remainder of walk.nbytes
modulo AES_BLOCK_SIZE with the residual nbytes, and jumping straight
into the tail processing block if they are equal. This tail processing
block checks whether nbytes != 0, and does nothing otherwise.

However, in case of an allocation failure in the blkcipher layer, we
may enter this code with walk.nbytes == 0, while nbytes > 0. In this
case, we should not dereference the source and destination pointers,
since they may be NULL. So instead of checking for nbytes != 0, check
for (walk.nbytes % AES_BLOCK_SIZE) != 0, which implies the former in
non-error conditions.

Fixes: 49788fe2 ("arm64/crypto: AES-ECB/CBC/CTR/XTS using ARMv8 NEON and Crypto Extensions")
Cc: stable@vger.kernel.org
Reported-by: xiakaixu <xiakaixu@huawei.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/arm64/crypto/aes-glue.c

@@ -216,7 +216,7 @@ static int ctr_encrypt(struct blkcipher_desc *desc, struct scatterlist *dst,
 		err = blkcipher_walk_done(desc, &walk,
 					  walk.nbytes % AES_BLOCK_SIZE);
 	}
-	if (nbytes) {
+	if (walk.nbytes % AES_BLOCK_SIZE) {
 		u8 *tdst = walk.dst.virt.addr + blocks * AES_BLOCK_SIZE;
 		u8 *tsrc = walk.src.virt.addr + blocks * AES_BLOCK_SIZE;
 		u8 __aligned(8) tail[AES_BLOCK_SIZE];