Re: [PATCH 1/3] KVM: SEV: snapshot the GHCB before accessing it

[Tom Lendacky]

On 8/4/23 12:33, Paolo Bonzini wrote:
> Validation of the GHCB is susceptible to time-of-check/time-of-use vulnerabilities.
> To avoid them, we would like to always snapshot the fields that are read in
> sev_es_validate_vmgexit(), and not use the GHCB anymore after it returns.
>
> This means:
>
> - invoking sev_es_sync_from_ghcb() before any GHCB access, including before
>    sev_es_validate_vmgexit()
>
> - snapshotting all fields including the valid bitmap and the sw_scratch field,
>    which are currently not caching anywhere.
>
> The valid bitmap is the first thing to be copied out of the GHCB; then,
> further accesses will use the copy in svm->sev_es.
>
> Fixes: 291bd20d5d88 ("KVM: SVM: Add initial support for a VMGEXIT VMEXIT")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

Reviewed-by: Tom Lendacky <thomas.lendacky@xxxxxxx>

> ---
>   arch/x86/kvm/svm/sev.c | 69 +++++++++++++++++++++---------------------
>   arch/x86/kvm/svm/svm.h | 26 ++++++++++++++++
>   2 files changed, 61 insertions(+), 34 deletions(-)