Re: [PATCH Part2 RFC v4 34/40] KVM: SVM: Add support to handle Page State Change VMGEXIT

[Brijesh Singh]

On 7/16/21 4:14 PM, Sean Christopherson wrote:
> On Wed, Jul 07, 2021, Brijesh Singh wrote:
> > +static unsigned long snp_handle_psc(struct vcpu_svm *svm, struct ghcb *ghcb)
> > +{
> > +	struct kvm_vcpu *vcpu = &svm->vcpu;
> > +	int level, op, rc = PSC_UNDEF_ERR;
> > +	struct snp_psc_desc *info;
> > +	struct psc_entry *entry;
> > +	gpa_t gpa;
> > +
> > +	if (!sev_snp_guest(vcpu->kvm))
> > +		goto out;
> > +
> > +	if (!setup_vmgexit_scratch(svm, true, sizeof(ghcb->save.sw_scratch))) {
> > +		pr_err("vmgexit: scratch area is not setup.\n");
> > +		rc = PSC_INVALID_HDR;
> > +		goto out;
> > +	}
> > +
> > +	info = (struct snp_psc_desc *)svm->ghcb_sa;
> > +	entry = &info->entries[info->hdr.cur_entry];
>
> Grabbing "entry" here is unnecessary and confusing.

Noted.

> > +
> > +	if ((info->hdr.cur_entry >= VMGEXIT_PSC_MAX_ENTRY) ||
> > +	    (info->hdr.end_entry >= VMGEXIT_PSC_MAX_ENTRY) ||
> > +	    (info->hdr.cur_entry > info->hdr.end_entry)) {
>
> There's a TOCTOU bug here if the guest uses the GHCB instead of a scratch area.
> If the guest uses the scratch area, then KVM makes a full copy into kernel memory.
> But if the guest uses the GHCB, then KVM maps the GHCB into kernel address space
> but doesn't make a full copy, i.e. the guest can modify the data while it's being
> processed by KVM.
Sure, I can make a full copy of the page-state change buffer.


> IIRC, Peter and I discussed the sketchiness of the GHCB mapping offline a few
> times, but determined that there were no existing SEV-ES bugs because the guest
> could only submarine its own emulation request.  But here, it could coerce KVM
> into running off the end of a buffer.
>
> I think you can get away with capturing cur_entry/end_entry locally, though
> copying the GHCB would be more robust.  That would also make the code a bit
> prettier, e.g.
>
> 	cur = info->hdr.cur_entry;
> 	end = info->hdr.end_entry;
>
> > +		rc = PSC_INVALID_ENTRY;
> > +		goto out;
> > +	}
> > +
> > +	while (info->hdr.cur_entry <= info->hdr.end_entry) {
>
> Make this a for loop?

Sure, I can use the for loop. IIRC, in previous review feedbacks I got 
the feeling that while() was preferred in the part1 so I used the 
similar approach here.

> 	for ( ; cur_entry < end_entry; cur_entry++)
>
> > +		entry = &info->entries[info->hdr.cur_entry];
>
> Does this need array_index_nospec() treatment?

I don't think so.

thanks