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

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Message-ID: <YPH2qRkkG6m0FT2X@google.com>
Date:   Fri, 16 Jul 2021 21:14:17 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [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;
> +}