[PATCH 1/1] KVM: arm64: vgic: Do not remove in-flight LPIs from AP list on disable

[zjamg <ndaugoing@gmail.com>]

From: Yuchao Zhang <ndaugoing@gmail.com>

By code inspection, a race condition and potential Use-After-Free/crash
exists between remote LPI disabling and local LR folding when EOImode==0.

Commit 6da5e537f5af ("KVM: arm64: vgic: Pick EOIcount deactivations from
AP-list tail") introduced tracking the last interrupt placed in a List
Register in per-CPU host data (*host_data_ptr(last_lr_irq)) and traverses
the remaining ap_list via list_for_each_entry_continue() in
vgic_v3_fold_lr_state().

However, an interrupt loaded into an LR can be an LPI. While vCPU-B is
running the guest, another vCPU-A can write to vCPU-B's redistributor
GICR_CTLR to clear EnableLPIs, which dispatches vgic_flush_pending_lpis().

vgic_flush_pending_lpis() unconditionally removes all LPIs from ap_list
without checking whether the interrupt is currently in an LR
(irq->on_lr):
- It calls list_del(&irq->ap_list), setting ap_list.next to LIST_POISON1,
  and drops the AP-list reference.
- If the LPI is unmapped or its translation cache was invalidated, the
  vgic_irq refcount drops to zero and the object is freed via RCU.
- Meanwhile, vCPU-B's per-CPU *host_data_ptr(last_lr_irq) cannot be
  cleared by remote vCPUs and is left dangling.

When vCPU-B subsequently exits the guest:
1. vgic_v3_fold_lr_state() resumes using the unlinked last_lr_irq.
2. list_for_each_entry_continue() unconditionally evaluates
   list_next_entry(irq, ap_list) during loop initialization, accessing
   LIST_POISON1 (or freed memory).
3. If eoicount > 0, it attempts guard(raw_spinlock)(&irq->irq_lock) on
   the poisoned address, leading to an immediate host kernel panic.

Fix this by:
1. In vgic_flush_pending_lpis(), do not remove LPIs that are currently
   in-flight in an LR (irq->on_lr == true). They will be naturally pruned
   by the owning vCPU's vgic_prune_ap_list() after LR folding.
2. In vgic_fold_state(), clear *host_data_ptr(last_lr_irq) after folding
   so that no stale pointer survives past guest execution.
3. In vgic_v3_fold_lr_state() and vgic_v2_fold_lr_state(), bail out
   immediately if eoicount is zero, avoiding unnecessary list_next_entry()
   evaluation.

Note: this closes the primary race (the last_lr_irq node itself is no
longer unlinkable while in-flight), but the fold traversal can still
race with a remote flush unlinking a subsequent non-LR node in the
ap_list tail. Fully closing that window needs the fold side to take
references before dropping locks (in the spirit of the prune-side fix
in commit 7258770e5814 ("KVM: arm64: vgic: Handle race between
interrupt affinity change and LPI disabling")) and is left as a
follow-up.

Fixes: 6da5e537f5af ("KVM: arm64: vgic: Pick EOIcount deactivations from AP-list tail")
Cc: stable@vger.kernel.org
Signed-off-by: Yuchao Zhang <ndaugoing@gmail.com>
---
 arch/arm64/kvm/vgic/vgic-v2.c |  3 +++
 arch/arm64/kvm/vgic/vgic-v3.c |  3 +++
 arch/arm64/kvm/vgic/vgic.c    | 10 +++++++---
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-v2.c b/arch/arm64/kvm/vgic/vgic-v2.c
index 7182f63fc938..7b6cd05ce32d 100644
--- a/arch/arm64/kvm/vgic/vgic-v2.c
+++ b/arch/arm64/kvm/vgic/vgic-v2.c
@@ -122,6 +122,9 @@ void vgic_v2_fold_lr_state(struct kvm_vcpu *vcpu)
 	for (int lr = 0; lr < vgic_cpu->vgic_v2.used_lrs; lr++)
 		vgic_v2_fold_lr(vcpu, cpuif->vgic_lr[lr]);
 
+	if (!eoicount)
+		return;
+
 	/* See the GICv3 equivalent for the EOIcount handling rationale */
 	list_for_each_entry_continue(irq, &vgic_cpu->ap_list_head, ap_list) {
 		u32 lr;
diff --git a/arch/arm64/kvm/vgic/vgic-v3.c b/arch/arm64/kvm/vgic/vgic-v3.c
index 726e20a1da6e..c6eb5d9dca2d 100644
--- a/arch/arm64/kvm/vgic/vgic-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-v3.c
@@ -155,6 +155,9 @@ void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu)
 	for (int lr = 0; lr < cpuif->used_lrs; lr++)
 		vgic_v3_fold_lr(vcpu, cpuif->vgic_lr[lr]);
 
+	if (!eoicount)
+		return;
+
 	/*
 	 * EOIMode=0: use EOIcount to emulate deactivation. We are
 	 * guaranteed to deactivate in reverse order of the activation, so
diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index b25303d9919f..0d7d75c3ac93 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -205,10 +205,12 @@ void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
 		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;
+			if (!irq->on_lr) {
+				list_del(&irq->ap_list);
+				irq->vcpu = NULL;
+				deleted |= vgic_put_irq_norelease(vcpu->kvm, irq);
+			}
 			raw_spin_unlock(&irq->irq_lock);
-			deleted |= vgic_put_irq_norelease(vcpu->kvm, irq);
 		}
 	}
 
@@ -873,6 +875,8 @@ static void vgic_fold_state(struct kvm_vcpu *vcpu)
 		vgic_v2_fold_lr_state(vcpu);
 	else
 		vgic_v3_fold_lr_state(vcpu);
+
+	*host_data_ptr(last_lr_irq) = NULL;
 }
 
 /* Requires the irq_lock to be held. */
-- 
2.53.0