KVM: SVM: Skip VMSA init in sev_es_init_vmcb() if pointer is NULL

Skip initializing the VMSA physical address in the VMCB if the VMSA is
NULL, which occurs during intrahost migration as KVM initializes the VMCB
before copying over state from the source to the destination (including
the VMSA and its physical address).

In normal builds, __pa() is just math, so the bug isn't fatal, but with
CONFIG_DEBUG_VIRTUAL=y, the validity of the virtual address is verified
and passing in NULL will make the kernel unhappy.

Fixes: 6defa24 ("KVM: SEV: Init target VMCBs in sev_migrate_from")
Cc: stable@vger.kernel.org
Cc: Peter Gonda <pgonda@google.com>
Reviewed-by: Peter Gonda <pgonda@google.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Link: https://lore.kernel.org/r/20230825022357.2852133-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

arch/x86/kvm/svm/sev.c

@@ -2975,9 +2975,12 @@ static void sev_es_init_vmcb(struct vcpu_svm *svm)
 	/*
 	 * An SEV-ES guest requires a VMSA area that is a separate from the
 	 * VMCB page. Do not include the encryption mask on the VMSA physical
-	 * address since hardware will access it using the guest key.
+	 * address since hardware will access it using the guest key.  Note,
+	 * the VMSA will be NULL if this vCPU is the destination for intrahost
+	 * migration, and will be copied later.
 	 */
-	svm->vmcb->control.vmsa_pa = __pa(svm->sev_es.vmsa);
+	if (svm->sev_es.vmsa)
+		svm->vmcb->control.vmsa_pa = __pa(svm->sev_es.vmsa);
 
 	/* Can't intercept CR register access, HV can't modify CR registers */
 	svm_clr_intercept(svm, INTERCEPT_CR0_READ);