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

[Peter Gonda]

On Fri, Aug 4, 2023 at 11:34 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> 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@xxxxxxxxxx>

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