Re: [PATCH] crypto: xts: use memmove to avoid overlapped memory copy

[Ard Biesheuvel <ardb@kernel.org>]

On Thu, 16 Jul 2020 at 18:29, Colin King <colin.king@canonical.com> wrote:
>
> From: Colin Ian King <colin.king@canonical.com>
>
> There is a memcpy that performs a potential overlapped memory copy
> from source b to destination b + 1.  Fix this by using the safer
> memmove instead.
>
> Addresses-Coverity: ("Overlapping buffer in memory copy")
> Fixes: 8083b1bf8163 ("crypto: xts - add support for ciphertext stealing")
> Signed-off-by: Colin Ian King <colin.king@canonical.com>
> ---
>  crypto/xts.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/crypto/xts.c b/crypto/xts.c
> index 3565f3b863a6..fa3e6e7b7043 100644
> --- a/crypto/xts.c
> +++ b/crypto/xts.c
> @@ -169,7 +169,7 @@ static int 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);
> +       memmove(b + 1, b, tail);

This is a false positive: tail is guaranteed to be smaller than
sizeof(*b), so memmove() is unnecessary here.

If changing to memcpy(&b[1], &b[0], tail) makes the warning go away, i
am fine with it, but otherwise we should just leave it as is.


>         scatterwalk_map_and_copy(b, req->src, offset, tail, 0);
>
>         le128_xor(b, &rctx->t, b);
> --
> 2.27.0
>