Re: [PATCH v2] selftests/sgx: Fix Q1 and Q2 calculation in sigstruct.c

[Shuah Khan]

On 7/26/21 9:12 PM, Jarkko Sakkinen wrote:
> On Fri, Jul 23, 2021 at 01:53:06PM -0600, Shuah Khan wrote:
> > On 7/4/21 11:09 PM, Jarkko Sakkinen wrote:
> > > From: Tianjia Zhang <tianjia.zhang@xxxxxxxxxxxxxxxxx>
> > >
> > > Q1 and Q2 are numbers with *maximum* length of 384 bytes. If the calculated
> > > length of Q1 and Q2 is less than 384 bytes, things will go wrong.
> > >
> > > E.g. if Q2 is 383 bytes, then
> > >
> > > 1. The bytes of q2 are copied to sigstruct->q2 in calc_q1q2().
> > > 2. The entire sigstruct->q2 is reversed, which results it being
> > >      256 * Q2, given that the last byte of sigstruct->q2 is added
> > >      to before the bytes given by calc_q1q2().
> > >
> > > Either change in key or measurement can trigger the bug. E.g. an unmeasured
> > > heap could cause a devastating change in Q1 or Q2.
> > >
> > > Reverse exactly the bytes of Q1 and Q2 in calc_q1q2() before returning to
> > > the caller.
> > >
> > > Fixes: dedde2634570 ("selftests/sgx: Trigger the reclaimer in the selftests")
> > > Link: https://lore.kernel.org/linux-sgx/20210301051836.30738-1-tianjia.zhang@xxxxxxxxxxxxxxxxx/
> > > Signed-off-by: Tianjia Zhang <tianjia.zhang@xxxxxxxxxxxxxxxxx>
> > > Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> > > ---
> > > The original patch did a bad job explaining the code change but it
> > > turned out making sense. I wrote a new description.
> > >
> > > v2:
> > > - Added a fixes tag.
> > >    tools/testing/selftests/sgx/sigstruct.c | 41 +++++++++++++------------
> > >    1 file changed, 21 insertions(+), 20 deletions(-)
> > >
> > > diff --git a/tools/testing/selftests/sgx/sigstruct.c b/tools/testing/selftests/sgx/sigstruct.c
> > > index dee7a3d6c5a5..92bbc5a15c39 100644
> > > --- a/tools/testing/selftests/sgx/sigstruct.c
> > > +++ b/tools/testing/selftests/sgx/sigstruct.c
> > > @@ -55,10 +55,27 @@ static bool alloc_q1q2_ctx(const uint8_t *s, const uint8_t *m,
> > >    	return true;
> > >    }
> > > +static void reverse_bytes(void *data, int length)
> > > +{
> > > +	int i = 0;
> > > +	int j = length - 1;
> > > +	uint8_t temp;
> > > +	uint8_t *ptr = data;
> > > +
> > > +	while (i < j) {
> > > +		temp = ptr[i];
> > > +		ptr[i] = ptr[j];
> > > +		ptr[j] = temp;
> > > +		i++;
> > > +		j--;
> > > +	}
> > > +}
> >
> > I was just about apply this one and noticed this reverse_bytes().
> > Aren't there byteswap functions you could call instead of writing
> > your own?
>
> Sorry for latency, just came from two week leave.
>
> glibc does provide bswap for 16, 32, 64 bit numbers but nothing better.
>   
> I have no idea if libssl has such function. Since the test code already
> uses this function, and it works, and it's not a newly added function in
> this patch, I would consider keeping it.
>   

I will queue this up since it is fixing an important problem.
Let's look into if this can be replaced with a lib call when
you do cleanups perhaps for the next release.

thanks,
-- Shuah