Re: [PATCH] KVM: SVM: Make sure GHCB is mapped before updating

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 07, 2021, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> The sev_vcpu_deliver_sipi_vector() routine will update the GHCB to inform
> the caller of the AP Reset Hold NAE event that a SIPI has been delivered.
> However, if a SIPI is performed without a corresponding AP Reset Hold,
> then the GHCB may not be mapped, which will result in a NULL pointer
> dereference.
> 
> Check that the GHCB is mapped before attempting the update.

It's tempting to say the ghcb_set_*() helpers should guard against this, but
that would add a lot of pollution and the vast majority of uses are very clearly
in the vmgexit path.  svm_complete_emulated_msr() is the only other case that
is non-obvious; would it make sense to sanity check svm->ghcb there as well?

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 019ac836dcd0..abe9c765628f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2728,7 +2728,8 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 static int svm_complete_emulated_msr(struct kvm_vcpu *vcpu, int err)
 {
        struct vcpu_svm *svm = to_svm(vcpu);
-       if (!sev_es_guest(vcpu->kvm) || !err)
+
+       if (!err || !sev_es_guest(vcpu->kvm) || !WARN_ON_ONCE(svm->ghcb))
                return kvm_complete_insn_gp(vcpu, err);

        ghcb_set_sw_exit_info_1(svm->ghcb, 1);

> Fixes: 647daca25d24 ("KVM: SVM: Add support for booting APs in an SEV-ES guest")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>

Either way:

Reviewed-by: Sean Christopherson <seanjc@google.com> 

> ---
>  arch/x86/kvm/svm/sev.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 83e00e524513..13758e3b106d 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2105,5 +2105,6 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>  	 * the guest will set the CS and RIP. Set SW_EXIT_INFO_2 to a
>  	 * non-zero value.
>  	 */
> -	ghcb_set_sw_exit_info_2(svm->ghcb, 1);
> +	if (svm->ghcb)
> +		ghcb_set_sw_exit_info_2(svm->ghcb, 1);
>  }
> -- 
> 2.31.0
>