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

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Date:   Mon, 14 Aug 2023 07:58:54 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     pgonda@...gle.com, seanjc@...gle.com, theflow@...gle.com,
        vkuznets@...hat.com
Subject: Re: [PATCH 0/3] KVM: SEV: only access GHCB fields once

On 8/4/23 12:33, Paolo Bonzini 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!

Sorry, just got back from vacation... I'll review this as soon as I can.

Thanks,
Tom

> 
> Paolo
> 
> Paolo Bonzini (3):
>    KVM: SEV: snapshot the GHCB before accessing it
>    KVM: SEV: only access GHCB fields once
>    KVM: SEV: remove ghcb variable declarations
> 
>   arch/x86/kvm/svm/sev.c | 124 ++++++++++++++++++++---------------------
>   arch/x86/kvm/svm/svm.h |  26 +++++++++
>   2 files changed, 87 insertions(+), 63 deletions(-)
>