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

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Message-ID: <dfca9007-c553-3724-ea52-ef86b919c9d6@canonical.com>
Date:   Thu, 16 Jul 2020 17:05:04 +0100
From:   Colin Ian King <colin.king@...onical.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        kernel-janitors@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] crypto: xts: use memmove to avoid overlapped memory copy

On 16/07/2020 16:56, Ard Biesheuvel wrote:
> On Thu, 16 Jul 2020 at 18:29, Colin King <colin.king@...onical.com> wrote:
>>
>> From: Colin Ian King <colin.king@...onical.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@...onical.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.

In that case, just leave it as is. Apologies for the noise.

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