[dm-devel] xts fuzz testing and lack of ciphertext stealing support

[Ard Biesheuvel]

On Fri, 9 Aug 2019 at 23:57, Pascal Van Leeuwen
<pvanleeuwen at verimatrix.com> wrote:
>
> > -----Original Message-----
> > From: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> > Sent: Friday, August 9, 2019 7:49 PM
> > To: Horia Geanta <horia.geanta at nxp.com>
> > Cc: Herbert Xu <herbert at gondor.apana.org.au>; Pascal Van Leeuwen
> > <pvanleeuwen at verimatrix.com>; Milan Broz <gmazyland at gmail.com>; dm-devel at redhat.com; linux-
> > crypto at vger.kernel.org
> > Subject: Re: [dm-devel] xts fuzz testing and lack of ciphertext stealing support
> >
> > On Fri, 9 Aug 2019 at 10:44, Horia Geanta <horia.geanta at nxp.com> wrote:
> > >
> > > On 8/9/2019 9:45 AM, Ard Biesheuvel wrote:
> > > > On Fri, 9 Aug 2019 at 05:48, Herbert Xu <herbert at gondor.apana.org.au> wrote:
> > > >>
> > > >> On Thu, Aug 08, 2019 at 06:01:49PM +0000, Horia Geanta wrote:
> > > >>>
> > > >>> -- >8 --
> > > >>>
> > > >>> Subject: [PATCH] crypto: testmgr - Add additional AES-XTS vectors for covering
> > > >>>  CTS (part II)
> > > >>
> > > >> Patchwork doesn't like it when you do this and it'll discard
> > > >> your patch.  To make it into patchwork you need to put the new
> > > >> Subject in the email headers.
> > > >>
> > > >
> > > > IMO, pretending that your XTS implementation is compliant by only
> > > I've never said that.
> > > Some parts are compliant, some are not.
> > >
> > > > providing test vectors with the last 8 bytes of IV cleared is not the
> > > > right fix for this issue. If you want to be compliant, you will need
> > > It's not a fix.
> > > It's adding test vectors which are not provided in the P1619 standard,
> > > where "data unit sequence number" is at most 5B.
> > >
> >
> > Indeed. But I would prefer not to limit ourselves to 5 bytes of sector
> > numbers in the test vectors. However, we should obviously not add test
> > vectors that are known to cause breakages on hardware that works fine
> > in practice.
> >
> Well, obviously, the full 16 byte sector number vectors fail on existing
> CAAM hardware, which I do assume to work fine in practice. And you know
> I'm not in favor of building all kinds of workarounds into the drivers.
>
> Fact is, we know there are no current users that need more than 64 bits
> of IV. Fact is also that having 64 bits of IV in the vectors is already
> an improvement over the 40 bits in the original vectors. And unlike CTS,
> I am not aware of any real use case for more than 64 bits.
> Finally, another fact is that limiting the *vectors* to 64 bits of IV
> does not prohibit anyone from *using* a full 128 bit IV on an
> implementation that *does* support this. I would think most users of
> XTS, like dmcrypt, would allow you to specify the cra_drivername
> explictly anyway, so just don't select legacy CAAM if you need that.
> (heck, if it would be reading and writing its own data, and not need
> compatibility with other implementations, it wouldn't even matter)
>
> So yes, the specs are quite clear on the sector number being a full
> 128 bits. But that doesn't prevent us from specifying that the
> crypto API implementation currently only supports 64 bits, with the
> remaining bits being forced to 0. We can always revisit that when
> an actual use case for more than 64 bits arises ...
>

You have got it completely backwards:

CTS has never worked in any kernel implementation, so regardless of
what the spec says, supporting it in the kernel is not a high priority
issue whichever way you put it. Now is the first time anyone has asked
for it in 12 years, and only because someone spotted a deviation
between a h/w and a s/w implementation, not because anyone tried to
use it and failed. (Note that passing anything other than a multiple
of the block size will cause an error rather than fail silently)

Truncated IVs are a huge issue, since we already expose the correct
API via AF_ALG (without any restrictions on how many of the IV bits
are populated), and apparently, if your AF_ALG request for xts(aes)
happens to be fulfilled by the CAAM driver and your implementation
uses more than 64 bits for the IV, the top bits get truncated silently
and your data might get eaten.

In my experience, users tend to care more about the latter than the former.


> > > > to provide a s/w fallback for these cases.
> > > >
> > > Yes, the plan is to:
> > >
> > > -add 16B IV support for caam versions supporting it - caam Era 9+,
> > > currently deployed in lx2160a and ls108a
> > >
> > > -remove current 8B IV support and add s/w fallback for affected caam versions
> > > I'd assume this could be done dynamically, i.e. depending on IV provided
> > > in the crypto request to use either the caam engine or s/w fallback.
> > >
> >
> > Yes. If the IV received from the caller has bytes 8..15 cleared, you
> > use the limited XTS h/w implementation, otherwise you fall back to
> > xts(ecb-aes-caam..).
>
> Regards,
> Pascal van Leeuwen
> Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
> www.insidesecure.com
>