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;
+}