[PATCH v4 3/8] KVM: arm64: Fix MTE flag initialization for protected VMs

[Fuad Tabba]

The function pkvm_init_features_from_host() initializes guest
features, propagating them from the host. The logic to propagate
KVM_ARCH_FLAG_MTE_ENABLED (Memory Tagging Extension)
has a couple of issues.

First, the check was in the common path, before the divergence for
protected and non-protected VMs. For non-protected VMs, this was
unnecessary, as 'kvm->arch.flags' is completely overwritten by
host_arch_flags immediately after, which already contains the MTE flag.
For protected VMs, this was setting the flag even if the feature is not
allowed.

Second, the check was reading 'host_kvm->arch.flags' instead of using
the local 'host_arch_flags', which is read once from the host flags.

Fix these by moving the MTE flag check inside the protected-VM-only
path, checking if the feature is allowed, and changing it to use the
correct host_arch_flags local variable. This ensures non-protected VMs
get the flag via the bulk copy, and protected VMs get it via an explicit
check.

Fixes: b7f345fbc32a ("KVM: arm64: Fix FEAT_MTE in pKVM")
Signed-off-by: Fuad Tabba <tabba at google.com>
---
 arch/arm64/kvm/hyp/nvhe/pkvm.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
index f6f8996c4f97..16d7bf493c18 100644
--- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
+++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
@@ -337,9 +337,6 @@ static void pkvm_init_features_from_host(struct pkvm_hyp_vm *hyp_vm, const struc
 	/* CTR_EL0 is always under host control, even for protected VMs. */
 	hyp_vm->kvm.arch.ctr_el0 = host_kvm->arch.ctr_el0;
 
-	if (test_bit(KVM_ARCH_FLAG_MTE_ENABLED, &host_kvm->arch.flags))
-		set_bit(KVM_ARCH_FLAG_MTE_ENABLED, &kvm->arch.flags);
-
 	/* No restrictions for non-protected VMs. */
 	if (!kvm_vm_is_protected(kvm)) {
 		hyp_vm->kvm.arch.flags = host_arch_flags;
@@ -354,6 +351,9 @@ static void pkvm_init_features_from_host(struct pkvm_hyp_vm *hyp_vm, const struc
 		return;
 	}
 
+	if (kvm_pvm_ext_allowed(KVM_CAP_ARM_MTE))
+		kvm->arch.flags |= host_arch_flags & BIT(KVM_ARCH_FLAG_MTE_ENABLED);
+
 	bitmap_zero(allowed_features, KVM_VCPU_MAX_FEATURES);
 
 	set_bit(KVM_ARM_VCPU_PSCI_0_2, allowed_features);
-- 
2.51.2.1041.gc1ab5b90ca-goog