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Date:   Fri, 16 Jul 2021 21:14:17 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Brijesh Singh <brijesh.singh@amd.com>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-efi@vger.kernel.org, platform-driver-x86@vger.kernel.org,
        linux-coco@lists.linux.dev, linux-mm@kvack.org,
        linux-crypto@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Joerg Roedel <jroedel@suse.de>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ard Biesheuvel <ardb@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sergio Lopez <slp@redhat.com>, Peter Gonda <pgonda@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
        David Rientjes <rientjes@google.com>,
        Dov Murik <dovmurik@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@ibm.com>,
        Borislav Petkov <bp@alien8.de>,
        Michael Roth <michael.roth@amd.com>,
        Vlastimil Babka <vbabka@suse.cz>, tony.luck@intel.com,
        npmccallum@redhat.com, brijesh.ksingh@gmail.com
Subject: Re: [PATCH Part2 RFC v4 34/40] KVM: SVM: Add support to handle Page
 State Change VMGEXIT
Message-ID: <YPH2qRkkG6m0FT2X@google.com>
References: <20210707183616.5620-1-brijesh.singh@amd.com>
 <20210707183616.5620-35-brijesh.singh@amd.com>
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In-Reply-To: <20210707183616.5620-35-brijesh.singh@amd.com>
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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;
> +}