linux-kernel - Re: [PATCH 0/3] KVM: SEV: only access GHCB fields once

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Message-ID: <CAMkAt6o1OGCXb1ET5qduLX=211f9SRs+CFyM86kHXZPZRN_KCg@mail.gmail.com>
Date:   Wed, 9 Aug 2023 08:38:10 -0600
From:   Peter Gonda <pgonda@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        seanjc@...gle.com, theflow@...gle.com, vkuznets@...hat.com,
        thomas.lendacky@....com
Subject: Re: [PATCH 0/3] KVM: SEV: only access GHCB fields once

On Fri, Aug 4, 2023 at 11:34 AM Paolo Bonzini <pbonzini@...hat.com> wrote:
>
> The VMGEXIT handler has a time-of-check/time-of-use vulnerability; due
> to a double fetch, the guest can exploit a race condition to invoke
> the VMGEXIT handler recursively.  It is extremely difficult to
> reliably win the race ~100 consecutive times in order to cause an
> overflow, and the impact is usually mitigated by CONFIG_VMAP_STACK,
> but it ought to be fixed anyway.
>
> One way to do so could be to snapshot the whole GHCB, but this is
> relatively expensive.  Instead, because the VMGEXIT handler already
> syncs the GHCB to internal KVM state, this series makes sure that the
> GHCB is not read outside sev_es_sync_from_ghcb().
>
> Patch 1 adds caching for fields that currently are not snapshotted
> in host memory; patch 2 ensures that the cached fields are always used,
> thus fixing the race.  Finally patch 3 removes some local variables
> that are prone to incorrect use, to avoid reintroducing the race in
> other places.
>
> Please review!
>

Tested-by: Peter Gonda <pgonda@...gle.com>

I booted an Ubuntu guest and ran our internal GHCB correctness test
with these patches.