Re: [PATCH v3] KVM: SVM: Make sure GHCB is mapped before updating

[Sean Christopherson]

On Fri, Apr 09, 2021, Tom Lendacky wrote:
> From: Tom Lendacky <thomas.lendacky@xxxxxxx>
> 
> 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@xxxxxxx>
> 
> ---

Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx>