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

[Sean Christopherson]

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>
---
 arch/x86/kvm/svm/sev.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index 9f1585f40c85..f4f5d554eaaa 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -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);
--
2.32.0.554.ge1b32706d8-goog

And fixup for this patch:


diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
index f3df1eba0c72..416ae0b687fc 100644
--- a/arch/x86/kvm/svm/sev.c
+++ b/arch/x86/kvm/svm/sev.c
@@ -180,13 +180,12 @@ static int sev_get_asid(struct kvm *kvm)
 static void sev_asid_free(struct kvm_sev_info *sev)
 {
 	struct svm_cpu_data *sd;
-	int cpu, pos;
+	int cpu;
 	enum misc_res_type type;
 
 	mutex_lock(&sev_bitmap_lock);
 
-	pos = sev->asid - 1;
-	__set_bit(pos, sev_reclaim_asid_bitmap);
+	__set_bit(sev->asid, sev_reclaim_asid_bitmap);
 
 	for_each_possible_cpu(cpu) {
 		sd = per_cpu(svm_data, cpu);
@@ -1928,7 +1927,7 @@ int sev_cpu_init(struct svm_cpu_data *sd)
 	if (!sev_enabled)
 		return 0;
 
-	sd->sev_vmcbs = kcalloc(max_sev_asid + 1, sizeof(void *), GFP_KERNEL);
+	sd->sev_vmcbs = kcalloc(nr_asids, sizeof(void *), GFP_KERNEL);
 	if (!sd->sev_vmcbs)
 		return -ENOMEM;
 
--