CVE-2021-47008

In the Linux kernel, the following vulnerability has been resolved: KVM:
SVM: Make sure GHCB is mapped before updating 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.