Re: xts fuzz testing and lack of ciphertext stealing support

[Eric Biggers <ebiggers@kernel.org>]

On Wed, Jul 17, 2019 at 05:09:31PM +0000, Horia Geanta wrote:
> On 7/17/2019 1:16 AM, Eric Biggers wrote:
> > Hi Horia,
> > 
> > On Tue, Jul 16, 2019 at 05:46:29PM +0000, Horia Geanta wrote:
> >> Hi,
> >>
> >> With fuzz testing enabled, I am seeing xts(aes) failures on caam drivers.
> >>
> >> Below are several failures, extracted from different runs:
> >>
> >> [    3.921654] alg: skcipher: xts-aes-caam encryption unexpectedly succeeded on test vector "random: len=40 klen=64"; expected_error=-22, cfg="random: inplace use_finup nosimd src_divs=[57.93%@+11, 37.18%@+164, <reimport>0.68%@+4, 0.50%@+305, 3.71%@alignmask+3975]" 
> >>
> >> [    3.726698] alg: skcipher: xts-aes-caam encryption unexpectedly succeeded on test vector "random: len=369 klen=64"; expected_error=-22, cfg="random: inplace may_sleep use_digest src_divs=[100.0%@alignmask+584]" 
> >>
> >> [    3.741082] alg: skcipher: xts-aes-caam encryption unexpectedly succeeded on test vector "random: len=2801 klen=64"; expected_error=-22, cfg="random: inplace may_sleep use_digest src_divs=[100.0%@+6] iv_offset=18"
> >>
> >> It looks like the problem is not in CAAM driver.
> >> More exactly, fuzz testing is generating random test vectors and running
> >> them through both SW generic (crypto/xts.c) and CAAM implementation:
> >> -SW generic implementation of xts(aes) does not support ciphertext stealing
> >> and throws -EINVAL when input is not a multiple of AES block size (16B)
> >> -caam has support for ciphertext stealing, and allows for any input size
> >> which results in "unexpectedly succeeded" error messages.
> >>
> >> Any suggestion how this should be fixed?
> >>
> >> Thanks,
> >> Horia
> > 
> > I don't think drivers should allow inputs the generic implementation doesn't,
> > since those inputs aren't tested by the crypto self-tests (so how do you know
> How about implementation adding static test vectors and using them to validate
> the standard feature set that's not supported by the generic implementation?
> 
> > it's even correct?), and people could accidentally rely on the driver-specific
> > behavior and then be unable to migrate to another platform or implementation.
> > 
> People could also *intentionally* rely on a driver offering an implementation
> that is closer to the spec / standard.
> 
> > So for now I recommend just updating the caam driver to return -EINVAL on XTS
> > inputs not evenly divisible by the block size.
> > 
> I was hoping for something more constructive...
> 
> > Of course, if there are actual use cases for XTS with ciphertext stealing in the
> > kernel, we could add it to all the other implementations too.  But I'm not aware
> > of any currently.  Don't all XTS users in the kernel pass whole blocks?
> > 
> That's my guess too.
> 
> What about user space relying on offloading and xts working
> according to the spec?
> 

Sure, AF_ALG users could expect ciphertext stealing to work.  I don't know of
any actual examples of people saying they want it, but it's possible.

My point is simply that we add this, we need to find a way to support it in all
implementations.  It's not helpful to add it to only one specific driver, as
then it's inconsistent and is untestable with the common tests.

- Eric