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

[Jarkko Sakkinen <jarkko@kernel.org>]

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@linux.alibaba.com>
> > 
> > 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@linux.alibaba.com/
> > Signed-off-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
> > Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>
> > ---
> > 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.
 
> thanks,
> -- Shuah

/Jarkko