[PATCH] KVM: arm64: Handle race between interrupt affinity change and LPI disabling

[Marc Zyngier]

Hyunwoo Kim reports some really bad races should the following
situation occur:

- LPI-I is pending in vcpu-B's AP list
- vcpu-A writes to vcpu-B's RD to disable its LPIs
- vcpu-C moves I from B to C

If the last two race nicely enough, vgic_prune_ap_list() can drop
the irq and AP list locks, reacquire them, and in the interval
the irq has been freed. UAF follows.

The fix is two-fold:

- Before dropping the irq and ap_list locks, take a reference on
  the irq

- Do not try to handle migration of the pending bit: there is no
  expectation that this state is retained, as per the architecture

With that, we're sure that the interrupt is still around, and we
safely remove it from the AP list as it has no target at this
stage (unless another interrupt fires, but that's another story).

Reported-by: Hyunwoo Kim <imv4bel at gmail.com>
Tested-by: Hyunwoo Kim <imv4bel at gmail.com>
Link: https://lore.kernel.org/r/ailsCnyoS82r_QRz@v4bel
Signed-off-by: Marc Zyngier <maz at kernel.org>
---
 arch/arm64/kvm/vgic/vgic.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index 5a4768d8cd4f3..70a161383e5a6 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -203,6 +203,7 @@ void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
 	list_for_each_entry_safe(irq, tmp, &vgic_cpu->ap_list_head, ap_list) {
 		if (irq_is_lpi(vcpu->kvm, irq->intid)) {
 			raw_spin_lock(&irq->irq_lock);
+			irq->pending_latch = false;
 			list_del(&irq->ap_list);
 			irq->vcpu = NULL;
 			raw_spin_unlock(&irq->irq_lock);
@@ -792,7 +793,11 @@ static void vgic_prune_ap_list(struct kvm_vcpu *vcpu)
 			continue;
 		}
 
-		/* This interrupt looks like it has to be migrated. */
+		/*
+		 * This interrupt looks like it has to be migrated,
+		 * make sure it is kept alive while locks are dropped.
+		 */
+		vgic_get_irq_ref(irq);
 
 		raw_spin_unlock(&irq->irq_lock);
 		raw_spin_unlock(&vgic_cpu->ap_list_lock);
@@ -836,6 +841,8 @@ static void vgic_prune_ap_list(struct kvm_vcpu *vcpu)
 		raw_spin_unlock(&vcpuB->arch.vgic_cpu.ap_list_lock);
 		raw_spin_unlock(&vcpuA->arch.vgic_cpu.ap_list_lock);
 
+		deleted_lpis |= vgic_put_irq_norelease(vcpu->kvm, irq);
+
 		if (target_vcpu_needs_kick) {
 			kvm_make_request(KVM_REQ_IRQ_PENDING, target_vcpu);
 			kvm_vcpu_kick(target_vcpu);
-- 
2.47.3