Re: [PATCH v2] KVM: SVM: Make sure GHCB is mapped before updating

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 08, 2021, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> Access to the GHCB is mainly in the VMGEXIT path and it is known that the
> GHCB will be mapped. But there are two paths where it is possible the GHCB
> might not be mapped.
> 
> 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 might not be mapped (depending on the previous VMEXIT),
> which will result in a NULL pointer dereference.
> 
> The svm_complete_emulated_msr() routine will update the GHCB to inform
> the caller of a RDMSR/WRMSR operation about any errors. While it is likely
> that the GHCB will be mapped in this situation, add a safe guard
> in this path to be certain a NULL pointer dereference is not encountered.
> 
> Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
> Fixes: 647daca25d24 ("KVM: SVM: Add support for booting APs in an SEV-ES guest")
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> 
> ---
> 
> Changes from v1:
> - Added the svm_complete_emulated_msr() path as suggested by Sean
>   Christopherson
> - Add a WARN_ON_ONCE() to the sev_vcpu_deliver_sipi_vector() path
> ---
>  arch/x86/kvm/svm/sev.c | 3 +++
>  arch/x86/kvm/svm/svm.c | 2 +-
>  2 files changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 83e00e524513..7ac67615c070 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -2105,5 +2105,8 @@ 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.
>  	 */
> +	if (WARN_ON_ONCE(!svm->ghcb))

Isn't this guest triggerable?  I.e. send a SIPI without doing the reset hold?
If so, this should not WARN.

> +		return;
> +
>  	ghcb_set_sw_exit_info_2(svm->ghcb, 1);
>  }
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 271196400495..534e52ba6045 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -2759,7 +2759,7 @@ 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);
> -- 
> 2.31.0
>