[PATCH] crypto: xts - Replace memcpy() invocation with simple assignment

[PATCH] crypto: xts - Replace memcpy() invocation with simple assignment

[Ard Biesheuvel]

Colin reports that the memcpy() call in xts_cts_final() trigggers a
"Overlapping buffer in memory copy" warning in Coverity, which is a
false postive, given that tail is guaranteed to be smaller than or
equal to the distance between source and destination.

However, given that any additional bytes that we copy will be ignored
anyway, we can simply copy XTS_BLOCK_SIZE unconditionally, which means
we can use struct assignment of the array members instead, which is
likely to be more efficient as well.

Addresses-Coverity: ("Overlapping buffer in memory copy")
Fixes: 8083b1bf8163 ("crypto: xts - add support for ciphertext stealing")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 crypto/xts.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/crypto/xts.c b/crypto/xts.c
index 3c3ed02c7663..ad45b009774b 100644
--- a/crypto/xts.c
+++ b/crypto/xts.c
@@ -171,7 +171,7 @@ static int xts_cts_final(struct skcipher_request *req,
 				      offset - XTS_BLOCK_SIZE);
 
 	scatterwalk_map_and_copy(b, rctx->tail, 0, XTS_BLOCK_SIZE, 0);
-	memcpy(b + 1, b, tail);
+	b[1] = b[0];
 	scatterwalk_map_and_copy(b, req->src, offset, tail, 0);
 
 	le128_xor(b, &rctx->t, b);
-- 
2.17.1

Re: [PATCH] crypto: xts - Replace memcpy() invocation with simple assignment

[Herbert Xu]

On Tue, Jul 21, 2020 at 09:05:54AM +0300, Ard Biesheuvel wrote:
> Colin reports that the memcpy() call in xts_cts_final() trigggers a
> "Overlapping buffer in memory copy" warning in Coverity, which is a
> false postive, given that tail is guaranteed to be smaller than or
> equal to the distance between source and destination.
> 
> However, given that any additional bytes that we copy will be ignored
> anyway, we can simply copy XTS_BLOCK_SIZE unconditionally, which means
> we can use struct assignment of the array members instead, which is
> likely to be more efficient as well.
> 
> Addresses-Coverity: ("Overlapping buffer in memory copy")
> Fixes: 8083b1bf8163 ("crypto: xts - add support for ciphertext stealing")
> Reported-by: Colin Ian King <colin.king@canonical.com>
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
>  crypto/xts.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt