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