Re: [PATCH Part2 RFC v4 34/40] KVM: SVM: Add support to handle Page State Change VMGEXIT
From: Sean Christopherson <seanjc@google.com>
Date: 2021-07-16 21:14:27
Also in:
kvm, linux-coco, linux-efi, linux-mm, lkml, platform-driver-x86
On Wed, Jul 07, 2021, Brijesh Singh wrote:
quoted hunk ↗ jump to hunk
+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.
quoted hunk ↗ jump to hunk
+ + 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;
quoted hunk ↗ jump to hunk
+ 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?
quoted hunk ↗ jump to hunk
+ 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; +}