[PATCH 5.4 01/27] KVM: SVM: Fix off-by-one indexing when nullifying last used SEV VMCB

[Greg Kroah-Hartman]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

[ Upstream commit 179c6c27bf487273652efc99acd3ba512a23c137 ]

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@xxxxxxx>
Cc: Brijesh Singh <brijesh.singh@xxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 arch/x86/kvm/svm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 7341d22ed04f..2a958dcc80f2 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1783,7 +1783,7 @@ static void __sev_asid_free(int asid)
 
 	for_each_possible_cpu(cpu) {
 		sd = per_cpu(svm_data, cpu);
-		sd->sev_vmcbs[pos] = NULL;
+		sd->sev_vmcbs[asid] = NULL;
 	}
 }
 
-- 
2.30.2