KVM: arm64: nv: Hold mmu_lock when invalidating VNCR SW-TLB before translating

When translating a VNCR translation fault, we start by marking the
current SW-managed TLB as invalid, so that we can populate it
in place. This is, however, done without the mmu_lock held.

A consequence of this is that another CPU dealing with TLBI
emulation can observe a translation still flagged as valid, but
with invalid walk results (such as pgshift being 0). Bad things
can result from this, such as a BUG() in pgshift_level_to_ttl().

Fix it by taking the mmu_lock for write to perform this local
invalidation, and use invalidate_vncr() instead of open-coding
the write to the 'valid' flag.

Fixes: 069a05e535 ("KVM: arm64: nv: Handle VNCR_EL2-triggered faults")
Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20250520144116.3667978-1-maz@kernel.org
Signed-off-by: Marc Zyngier <maz@kernel.org>

arch/arm64/kvm/nested.c

@@ -1179,13 +1179,24 @@ static int kvm_translate_vncr(struct kvm_vcpu *vcpu)
 
 	vt = vcpu->arch.vncr_tlb;
 
-	vt->wi = (struct s1_walk_info) {
-		.regime	= TR_EL20,
-		.as_el0	= false,
-		.pan	= false,
-	};
-	vt->wr = (struct s1_walk_result){};
-	vt->valid = false;
+	/*
+	 * If we're about to walk the EL2 S1 PTs, we must invalidate the
+	 * current TLB, as it could be sampled from another vcpu doing a
+	 * TLBI *IS. A real CPU wouldn't do that, but we only keep a single
+	 * translation, so not much of a choice.
+	 *
+	 * We also prepare the next walk wilst we're at it.
+	 */
+	scoped_guard(write_lock, &vcpu->kvm->mmu_lock) {
+		invalidate_vncr(vt);
+
+		vt->wi = (struct s1_walk_info) {
+			.regime	= TR_EL20,
+			.as_el0	= false,
+			.pan	= false,
+		};
+		vt->wr = (struct s1_walk_result){};
+	}
 
 	guard(srcu)(&vcpu->kvm->srcu);