linux-kernel - Re: [PATCH Part2 v5 44/45] KVM: SVM: Support SEV-SNP AP Creation NAE event

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Message-ID: <YXCfxyfQBix8+As+@google.com>
Date:   Wed, 20 Oct 2021 23:01:27 +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-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>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part2 v5 44/45] KVM: SVM: Support SEV-SNP AP Creation NAE
 event

On Wed, Oct 20, 2021, Brijesh Singh wrote:
> 
> On 10/15/21 2:50 PM, Sean Christopherson wrote:
> > And digging through the guest patches, this gives the guest _full_ control over
> > the VMSA contents.  That is bonkers.  At _best_ it gives the guest the ability to
> > fuzz VMRUN ucode by stuffing garbage into the VMSA.
> 
> If guest puts garbage in VMSA then VMRUN will fail. I am sure ucode is
> doing all kind of sanity checks to ensure that VMSA does not contain
> invalid value before the run.

Oh, I'm well aware of the number of sanity checks that are in VM-Enter ucode, and
that's precisely why I'm of the opinion that letting the guest fuzz VMRUN is a
non-trivial security risk for the host.  I know of at least at least two VMX bugs
(one erratum that I could find, one that must have been fixed with a ucode patch?)
where ucode failed to detect invalid state.  Those were "benign" in that they
caused a missed VM-Fail but didn't corrupt CPU state, but it's not a stretch to
imagine a ucode bug that leads to corruption of CPU state and a system crash.

The sheer number of checks involved, combined with the fact that there likely
hasn't been much fuzzing of VM-Enter outside of the hardware vendor's own
validation, means I'm not exactly brimming with confidence that VMRUN's ucode
is perfect.

I fully acknowledge that the host kernel obviously "trusts" CPU ucode to a great
extent.  My point here is that the design exposes the host to unnecessary risk.