Re: [PATCH V4] virt: sev: Prevent IV reuse in SNP guest driver

[Tom Lendacky]

On 11/16/22 10:23, Peter Gonda wrote:
> On Wed, Nov 16, 2022 at 5:20 AM Borislav Petkov <bp@xxxxxxx> wrote:
> > On Tue, Nov 15, 2022 at 02:47:31PM -0700, Peter Gonda wrote:
> > > > > +      * certificate data buffer retry the same guest request without the
> > > > > +      * extended data request.
> > > > > +      */
> > > > > +     if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
> > > > > +         err == SNP_GUEST_REQ_INVALID_LEN) {
> > > > > +             const unsigned int certs_npages = snp_dev->input.data_npages;
> > > > > +
> > > > > +             exit_code = SVM_VMGEXIT_GUEST_REQUEST;
> > > > > +             rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
> > > > > +
> > > > > +             err = SNP_GUEST_REQ_INVALID_LEN;
> > > >
> > > > Huh, why are we overwriting err here?
> > >
> > > I have added a comment for the next revision.
> > >
> > > We are overwriting err here so that userspace is alerted that they
> > > supplied a buffer too small.
> >
> > Sure but you're not checking rc either. What if that reissue fails for
> > whatever other reason? -EIO for example...
>
> If we get any error here we have to wipe the VMPCK here so I thought

More accurate to say that you will wipe the VMPCK, since the value of rc 
is checked a bit further down in the code and the -EIO (or other non-zero) 
will be result in a call to snp_disable_vmpck() and rc being propagated 
back to the user as an ioctl() return code.

Might be worth a comment above that second snp_issue_guest_request() 
explaining that.

> this always override @err was OK.
>
> I can update this to only override @err if after the secondary
> SVM_VMGEXIT_GUEST_REQUEST rc and err are OK. Thoughts?

I think it's ok to set it no matter what, but I don't have a strong 
opinion either way.

Thanks,
Tom

> > --
> > Regards/Gruss,
> >      Boris.
> >
> > SUSE Software Solutions Germany GmbH
> > GF: Ivo Totev, Andrew Myers, Andrew McDonald, Martje Boudien Moerman
> > (HRB 36809, AG Nürnberg)