Re: [PATCH Part1 v5 34/38] x86/sev: Add snp_msg_seqno() helper

Peter Gonda
2021-09-09T16:21:36Z

On Thu, Sep 9, 2021 at 10:17 AM Brijesh Singh <brijesh.singh@amd.com> wrote:
>
>
>
> On 9/9/21 10:43 AM, Peter Gonda wrote:
> ...
>
> >>
> >> Does this address your concern?
> >
> > So the 'snp_msg_seqno()' call in 'enc_payload' will not increment the
> > counter, its only incremented on 'snp_gen_msg_seqno()'? If thats
> > correct, that addresses my first concern.
> >
>
> Yes, that is goal.
>
> >>>
> >>
> >> So far, the only user for the snp_msg_seqno() is the attestation driver.
> >> And the driver is designed to serialize the vmgexit request and thus we
> >> should not run into concurrence issue.
> >
> > That seems a little dangerous as any module new code or out-of-tree
> > module could use this function thus revealing this race condition
> > right? Could we at least have a comment on these functions
> > (snp_msg_seqno and snp_gen_msg_seqno) noting this?
> >
>
> Yes, if the driver is not performing the serialization then we will get
> into race condition.
>
> One way to avoid this requirement is to do all the crypto inside the
> snp_issue_guest_request() and eliminate the need to export the
> snp_msg_seqno().
>
> I will add the comment about it in the function.

Actually I forgot that the sequence number is the only component of
the AES-GCM IV. Seen in 'enc_payload'. Given the AES-GCM spec requires
uniqueness of the IV. I think we should try a little harder than a
comment to guarantee we never expose 2 requests encrypted with the
same sequence number / IV. It's more than just a DOS against the
guest's PSP request ability but also could be a guest security issue,
thoughts?

https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf
(Section 8 page 18)

>
> thanks