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

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Date:   Tue, 25 Sep 2018 18:16:39 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     Kees Cook <keescook@...omium.org>,
        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 Tue, 25 Sep 2018 at 18:03, Jens Axboe <axboe@...nel.dk> wrote:
>
> 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.
>

As far as I can tell, this function is only called from
lo_read_transfer/lo_write_transfer, both of which do an unconditional
alloc_page(GFP_NOIO), which is why I assumed that kmalloc(GFP_NOIO)
would be permissible in the same context. Are you saying this may not
be the case?