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

[Pascal Van Leeuwen <pvanleeuwen@verimatrix.com>]

> -----Original Message-----
> From: Eric Biggers <ebiggers@kernel.org>
> Sent: Sunday, August 11, 2019 10:34 PM
> To: Milan Broz <gmazyland@gmail.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>; Pascal Van Leeuwen
> <pvanleeuwen@verimatrix.com>; dm-devel@redhat.com; Herbert Xu <herbert@gondor.apana.org.au>;
> Horia Geanta <horia.geanta@nxp.com>; linux-crypto@vger.kernel.org
> Subject: Re: [dm-devel] xts fuzz testing and lack of ciphertext stealing support
> 
> On Sun, Aug 11, 2019 at 01:12:56PM +0200, Milan Broz wrote:
> > On 10/08/2019 06:39, Ard Biesheuvel wrote:
> > > 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.
> >
> > Actually, I think we have already serious problem with in in kernel (no AF_ALG needed).
> >
> > I do not have the hardware, but please could you check that dm-crypt big-endian IV
> > (plain64be) produces the same output on CAAM?
> >
> > It is 64bit IV, but big-endian and we use size of cipher block (16bytes) here,
> > so the first 8 bytes are zero in this case.
> >
> > I would expect data corruption in comparison to generic implementation,
> > if it supports only the first 64bit...
> >
> > Try this:
> >
> > # create small null device of 8 sectors,  we use zeroes as fixed ciphertext
> > dmsetup create zero --table "0 8 zero"
> >
> > # create crypt device on top of it (with some key), using plain64be IV
> > dmsetup create crypt --table "0 8 crypt aes-xts-plain64be
> e8cfa3dbfe373b536be43c5637387786c01be00ba5f730aacb039e86f3eb72f3 0 /dev/mapper/zero 0"
> >
> > # and compare it with and without your driver, this is what I get here:
> > # sha256sum /dev/mapper/crypt
> > 532f71198d0d84d823b8e410738c6f43bc3e149d844dd6d37fa5b36d150501e1  /dev/mapper/crypt
> > # dmsetup remove crypt
> >
> > You can try little-endian version (plain64), this should always work even with CAAM
> > dmsetup create crypt --table "0 8 crypt aes-xts-plain64
> e8cfa3dbfe373b536be43c5637387786c01be00ba5f730aacb039e86f3eb72f3 0 /dev/mapper/zero 0"
> >
> > # sha256sum /dev/mapper/crypt
> > f17abd27dedee4e539758eabdb6c15fa619464b509cf55f16433e6a25da42857  /dev/mapper/crypt
> > # dmsetup remove crypt
> >
> > # dmsetup remove zero
> >
> >
> > If you get different plaintext in the first case, your driver is actually creating
> > data corruption in this configuration and it should be fixed!
> > (Only the first sector must be the same, because it has IV == 0.)
> >
> > Milan
> >
> > p.s.
> > If you ask why we have this IV, it was added per request to allow map some chipset-based
> > encrypted drives directly. I guess it is used for some data forensic things.
> >
> 
> Also, if the CAAM driver is really truncating the IV for "xts(aes)", it should
> already be failing the extra crypto self-tests, since the fuzz testing in
> test_skcipher_vs_generic_impl() uses random IVs.
> 
> - Eric
>
Yes, good point. Although that is only seen during development and not 
during "normal" use ...


Regards,
Pascal van Leeuwen
Silicon IP Architect, Multi-Protocol Engines @ Verimatrix
www.insidesecure.com