linux-kernel - Re: [PATCH crypto-next 07/23] block: cryptoloop: Remove VLA usage of skcipher

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Date:   Tue, 25 Sep 2018 10:03:27 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kees Cook <keescook@...omium.org>
Cc:     Herbert Xu <herbert@...dor.apana.org.au>,
        linux-block@...r.kernel.org, Eric Biggers <ebiggers@...gle.com>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH crypto-next 07/23] block: cryptoloop: Remove VLA usage of
 skcipher

On 9/25/18 3:25 AM, Ard Biesheuvel wrote:
> On Mon, 24 Sep 2018 at 19:53, Kees Cook <keescook@...omium.org> wrote:
>>
>> On Mon, Sep 24, 2018 at 4:52 AM, Ard Biesheuvel
>> <ard.biesheuvel@...aro.org> wrote:
>>> On Wed, 19 Sep 2018 at 04:11, Kees Cook <keescook@...omium.org> wrote:
>>>> @@ -119,7 +119,7 @@ cryptoloop_transfer(struct loop_device *lo, int cmd,
>>>>         unsigned in_offs, out_offs;
>>>>         int err;
>>>>
>>>> -       skcipher_request_set_tfm(req, tfm);
>>>> +       skcipher_request_set_sync_tfm(req, tfm);
>>>>         skcipher_request_set_callback(req, CRYPTO_TFM_REQ_MAY_SLEEP,
>>>>                                       NULL, NULL);
>>>>
>>>
>>> Does this work?
>>
>> Everything is a direct wrapper for existing types and functions, so I
>> wouldn't expect any functional change. I haven't been able to test
>> this particular interface, though. cryptoloop is very deprecated,
>> isn't it?
>>
> 
> Ah yes, I managed to confuse myself there. This looks all fine to me.
> 
> In any case, this is another example where we may decide to fix the
> code rather than retain the request allocation on the stack (but that
> is Jens's call ultimately, I suppose)
> 
> diff --git a/drivers/block/cryptoloop.c b/drivers/block/cryptoloop.c
> index 7033a4beda66..5ed2167219ba 100644
> --- a/drivers/block/cryptoloop.c
> +++ b/drivers/block/cryptoloop.c
> @@ -110,7 +110,7 @@ cryptoloop_transfer(struct loop_device *lo, int cmd,
>                     int size, sector_t IV)
>  {
>         struct crypto_skcipher *tfm = lo->key_data;
> -       SKCIPHER_REQUEST_ON_STACK(req, tfm);
> +       struct skcipher_request *req;
>         struct scatterlist sg_out;
>         struct scatterlist sg_in;
> 
> @@ -119,7 +119,10 @@ cryptoloop_transfer(struct loop_device *lo, int cmd,
>         unsigned in_offs, out_offs;
>         int err;
> 
> -       skcipher_request_set_tfm(req, tfm);
> +       req = skcipher_request_alloc(tfm, GFP_NOIO);
> +       if (!req)
> +               return -ENOMEM;

Is this going to be reliable? ->transfer() is called when we're doing IO,
and you'd normally need a mempool backed allocation to make this safe
and guarantee forward progress.

-- 
Jens Axboe