[PATCH 5.13 148/175] KVM: SVM: Fix off-by-one indexing when nullifying last used SEV VMCB

From: Sean Christopherson <seanjc@google.com>

commit 179c6c27bf487273652efc99acd3ba512a23c137 upstream.

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.

Fixes: 70cd94e60c73 ("KVM: SVM: VMRUN should use associated ASID when SEV is enabled")
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Brijesh Singh <redacted>
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kvm/svm/sev.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c

@@ -188,7 +188,7 @@ static void sev_asid_free(struct kvm_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);