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

[Brijesh Singh]

On 9/9/21 11:21 AM, Peter Gonda wrote:
> On Thu, Sep 9, 2021 at 10:17 AM Brijesh Singh <brijesh.singh@xxxxxxx> 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?

Ah good point, we should avoid a request with same IV. May be move the 
sequence number increment and save in sevguest drv. Then driver can do 
the sequence get, vmgexit and increment with a protected lock.

thanks

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>
> > thanks