| Date | Fri, 16 Jul 2021 21:14:17 +0000 | From | Sean Christopherson <> | Subject | Re: [PATCH Part2 RFC v4 34/40] KVM: SVM: Add support to handle Page State Change VMGEXIT |
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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.
> + > + 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.
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?
for ( ; cur_entry < end_entry; cur_entry++)
> + entry = &info->entries[info->hdr.cur_entry];
Does this need array_index_nospec() treatment?
> + gpa = gfn_to_gpa(entry->gfn); > + level = RMP_TO_X86_PG_LEVEL(entry->pagesize); > + op = entry->operation; > + > + if (!IS_ALIGNED(gpa, page_level_size(level))) { > + rc = PSC_INVALID_ENTRY; > + goto out; > + } > + > + rc = __snp_handle_psc(vcpu, op, gpa, level); > + if (rc) > + goto out; > + > + info->hdr.cur_entry++; > + } > + > +out:
And for the copy case:
info->hdr.cur_entry = cur;
> + return rc ? map_to_psc_vmgexit_code(rc) : 0; > +}
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