FAILED: patch "[PATCH] KVM: SVM: Fix off-by-one indexing when nullifying last used" failed to apply to 4.19-stable tree

The patch below does not apply to the 4.19-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 179c6c27bf487273652efc99acd3ba512a23c137 Mon Sep 17 00:00:00 2001
From: Sean Christopherson <[email protected]>
Date: Tue, 3 Aug 2021 09:27:46 -0700
Subject: [PATCH] KVM: SVM: Fix off-by-one indexing when nullifying last used
SEV VMCB

Use the raw ASID, not ASID-1, when nullifying the last used VMCB when freeing an SEV ASID. The consumer, pre_sev_run(), indexes the array by the raw ASID, thus KVM could get a false negative when checking for a different VMCB if KVM manages to reallocate the same ASID+VMCB combo for a new VM.

Note, this cannot cause a functional issue _in the current code_, as pre_sev_run() also checks which pCPU last did VMRUN for the vCPU, and last_vmentry_cpu is initialized to -1 during vCPU creation, i.e. is guaranteed to mismatch on the first VMRUN. However, prior to commit 8a14fe4f0c54 ("kvm: x86: Move last_cpu into kvm_vcpu_arch as last_vmentry_cpu"), SVM tracked pCPU on its own and zero-initialized the last_cpu variable. Thus it's theoretically possible that older versions of KVM could miss a TLB flush if the first VMRUN is on pCPU0 and the ASID and VMCB exactly match those of a prior VM.

index 6710d9ee2e4b..4d0aba185412 100644
@@ -189,7 +189,7 @@ static void sev_asid_free(struct kvm_sev_info *sev)
 
 	for_each_possible_cpu(cpu) {
 		sd = per_cpu(svm_data, cpu);
-		sd->sev_vmcbs[pos] = NULL;
+		sd->sev_vmcbs[sev->asid] = NULL;
 	}
 
 	mutex_unlock(&sev_bitmap_lock);