Re: [PATCH v6 40/42] virt: Add SEV-SNP guest driver

[Brijesh Singh]

On 10/27/21 4:05 PM, Peter Gonda wrote:
....

> > > > > Thanks for updating this sequence number logic. But I still have some
> > > > > concerns. In verify_and_dec_payload() we check the encryption header
> > > > > but all these fields are accessible to the hypervisor, meaning it can
> > > > > change the header and cause this sequence number to not get
> > > > > incremented. We then will reuse the sequence number for the next
> > > > > command, which isn't great for AES GCM. It seems very hard to tell if
> > > > > the FW actually got our request and created a response there by
> > > > > incrementing the sequence number by 2, or if the hypervisor is acting
> > > > > in bad faith. It seems like to be safe we need to completely stop
> > > > > using this vmpck if we cannot confirm the PSP has gotten our request
> > > > > and created a response. Thoughts?
> > > >
> > > > Very good point, I think we can detect this condition by rearranging the
> > > > checks. The verify_and_dec_payload() is called only after the command is
> > > > succesful and does the following checks
> > > >
> > > > 1) Verifies the header
> > > > 2) Decrypts the payload
> > > > 3) Later we increment the sequence
> > > >
> > > > If we arrange to the below order then we can avoid this condition.
> > > > 1) Decrypt the payload
> > > > 2) Increment the sequence number
> > > > 3) Verify the header
> > > >
> > > > The descryption will succeed only if PSP constructed the payload.
> > > >
> > > > Does this make sense ?
> > >
> > > Either ordering seems fine to me. I don't think it changes much though
> > > since the header (bytes 30-50 according to the spec) are included in
> > > the authenticated data of the encryption. So any hypervisor modictions
> > > will lead to a decryption failure right?
> > >
> > > Either case if we do fail the decryption, what are your thoughts on
> > > not allowing further use of that VMPCK?
> >
> > We have limited number of VMPCK (total 3). I am not sure switching to
> > different will change much. HV can quickly exaust it. Once we have SVSM
> > in-place then its possible that SVSM may use of the VMPCK. If the
> > decryption failed, then maybe its safe to erase the key from the secrets
> > page (in other words guest OS cannot use that key for any further
> > communication). A guest can reload the driver will different VMPCK id
> > and try again.
>
> SNP cannot really cover DOS at all since the VMM could just never
> schedule the VM. In this case we know that the hypervisor is trying to
> mess with the guest, so my preference would be to stop sending guest
> messages to prevent that duplicated IV usage. If one caller gets an
> EBADMSG it knows its in this case but the rest of userspace has no
> idea. Maybe log an error?
>
> >

Yap, we cannot protect against the DOS. This is why I was saying that we 
zero the key from secrets page so that guest cannot use that key for any 
future communication (whether its from rest of userspace or kexec 
kernel). I can update the driver to log the message and ensure that 
future messages will *not* use that key. The VMPCK ID is a module 
params, so a guest can reload the driver to use different VMPCK.


> > thanks