linux-kernel - Re: [PATCH] KVM: SEV: Replace KVM_EXIT_VMGEXIT with KVM_EXIT_SNP_REQ

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Message-ID: <CAAH4kHaCGraqmD8Zi6CtFzYFBvg5vgaQEc_DYJ7PayONp22B-w@mail.gmail.com>
Date: Fri, 16 Aug 2024 14:58:10 -0700
From: Dionna Amalie Glaze <dionnaglaze@...gle.com>
To: Michael Roth <michael.roth@....com>
Cc: seanjc@...gle.com, kvm@...r.kernel.org, linux-coco@...ts.linux.dev, 
	linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org, 
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	pankaj.gupta@....com, liam.merwick@...cle.com, papaluri@....com
Subject: Re: [PATCH] KVM: SEV: Replace KVM_EXIT_VMGEXIT with KVM_EXIT_SNP_REQ_CERTS

> How do we avoid this?
> 1. We can advise that the guest parses the certificate and the
> attestation report to determine if their TCBs match expectations and
> retry if they're different because of a bad luck data race.
> 2. We can add a new global lock that KVM holds from CCP similar to
> sev_cmd_lock to sequentialize req_certs, attestation reports, and
> SNP_COMMIT. KVM releases the lock before returning to the guest.
>   SNP_COMMIT must now hold this lock before attempting to grab the sev_cmd_lock.
>
> I think probably 2 is better.
>

Actually no, we shouldn't hold a global lock and only release it if
user space returns to KVM in a specific way, unless we can ensure it
will be unlocked safely on fd close.

-- 
-Dionna Glaze, PhD, CISSP, CCSP (she/her)