[PATCH v2 0/1] KVM: arm64: vgic: fix UAF/crash on remote LPI disable

[PATCH v2 0/1] KVM: arm64: vgic: fix UAF/crash on remote LPI disable

[zjamg]

From: Yuchao Zhang <ndaugoing@gmail.com>

Hi Marc, Oliver, and KVM/arm64 maintainers,

By code inspection of commit 6da5e537f5af ("KVM: arm64: vgic: Pick EOIcount
deactivations from AP-list tail"), a race condition exists when a remote
vCPU disables LPIs while the target vCPU has an in-flight LPI in a List
Register (LR).

Specifically:
- vgic_flush_pending_lpis() unconditionally unlinks all LPIs from ap_list
  without checking whether the interrupt is in an LR (irq->on_lr).
- If the LPI in the LR happened to be the last one populated, the per-CPU
  pointer *host_data_ptr(last_lr_irq) on the target vCPU is left dangling.
- When the target vCPU exits guest mode, vgic_v3_fold_lr_state() starts
  traversing ap_list via list_for_each_entry_continue() from this unlinked,
  poisoned (or freed) last_lr_irq, leading to UAF or an immediate panic
  when locking irq->irq_lock.

Solution & Scope:
This patch prevents unlinking LPIs that are currently on an LR in
vgic_flush_pending_lpis(), ensures *host_data_ptr(last_lr_irq) is cleared
after folding, skips the ap_list walk when eoicount is zero, and prevents
the fold from resurrecting the pending state of an edge LPI once the
redistributor has LPIs disabled.

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.

Changes in v2:
- Added the fold-side guard: vgic_v3_fold_lr() no longer preserves the
  pending bit of an edge LPI folded while the redistributor has LPIs
  disabled. Without this, a flushed in-flight LPI is resurrected from
  the LR pending bit and re-injected while GICR_CTLR.EnableLPIs is 0
  (spurious LPI delivery to the guest), since the injection path has no
  lpis_enabled gate. Thanks to the Sashiko AI review for pointing this
  out.

Yuchao Zhang (1):
  KVM: arm64: vgic: Do not remove in-flight LPIs from AP list on disable

 arch/arm64/kvm/vgic/vgic-v2.c |  3 +++
 arch/arm64/kvm/vgic/vgic-v3.c | 14 ++++++++++++--
 arch/arm64/kvm/vgic/vgic.c    | 10 +++++++---
 3 files changed, 22 insertions(+), 5 deletions(-)

--
2.53.0

[PATCH v2] KVM: arm64: vgic: Do not remove in-flight LPIs from AP list on disable

[zjamg]

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.
4. In vgic_v3_fold_lr(), do not resurrect the pending state of an edge
   LPI folded while the redistributor has LPIs disabled.
   vgic_flush_pending_lpis() cannot remotely clear the LRs of a running
   vCPU, so such an LPI would otherwise be requeued and re-injected
   while GICR_CTLR.EnableLPIs is 0, violating the flush semantics (and
   the architecture, which gives no expectation of the pending state
   being retained across a disable).

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 | 14 ++++++++++++--
 arch/arm64/kvm/vgic/vgic.c    | 10 +++++++---
 3 files changed, 22 insertions(+), 5 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..dd1314f4543d 100644
--- a/arch/arm64/kvm/vgic/vgic-v3.c
+++ b/arch/arm64/kvm/vgic/vgic-v3.c
@@ -96,9 +96,16 @@ static void vgic_v3_fold_lr(struct kvm_vcpu *vcpu, u64 val)
 		deactivated = irq->active && !(val & ICH_LR_ACTIVE_BIT);
 		irq->active = !!(val & ICH_LR_ACTIVE_BIT);
 
-		/* Edge is the only case where we preserve the pending bit */
+		/*
+		 * Edge is the only case where we preserve the pending bit.
+		 * Do not resurrect the pending state of LPIs once the
+		 * redistributor has them disabled: vgic_flush_pending_lpis()
+		 * has discarded them, and the LRs of a running vCPU cannot
+		 * be remotely cleared.
+		 */
 		if (irq->config == VGIC_CONFIG_EDGE &&
-		    (val & ICH_LR_PENDING_BIT))
+		    (val & ICH_LR_PENDING_BIT) &&
+		    (irq->intid < VGIC_MIN_LPI || vgic_lpis_enabled(vcpu)))
 			irq->pending_latch = true;
 
 		/*
@@ -155,6 +162,9 @@ 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

Re: [PATCH v2] KVM: arm64: vgic: Do not remove in-flight LPIs from AP list on disable

[Fuad Tabba]

Hi Yuchao,

On Fri, 18 Sep 2026 12:02:14 +0800, Yuchao Zhang <ndaugoing@gmail.com> wrote:

[...]

> 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.

This is the same race Hyunwoo reported back in June [1]. You might
want to have a look at that thread first: Oliver and Marc's view there
was that the fix is to take the ap_list_lock in
vgic_v3_fold_lr_state() [2][3], and Hyunwoo posted a draft of that
[4].

Cheers,
/fuad

[1] https://lore.kernel.org/r/aiHrGM1f8czcUby4@v4bel
[2] https://lore.kernel.org/r/aiJi5a3JJ-TbWL-s@kernel.org
[3] https://lore.kernel.org/r/87a4t99z9n.wl-maz@kernel.org
[4] https://lore.kernel.org/r/aiXvwGD1hS6vwLEd@v4bel

Re: [PATCH v2] KVM: arm64: vgic: Do not remove in-flight LPIs from AP list on disable

[Yuchao Zhang]

Hi Fuad,

Thanks a lot for pointing me to that thread! I was not aware of
Hyunwoo's earlier report and the discussion with Oliver and Marc.

I'll read through the thread and their rationale on the ap_list_lock
approach. I'm happy to defer to Hyunwoo's effort to avoid duplicate
work.

Thanks again for the pointer!

Best regards,
Yuchao

Fuad Tabba <fuad.tabba@linux.dev> 于2026年9月18日周五 15:22写道：
>
> Hi Yuchao,
>
> On Fri, 18 Sep 2026 12:02:14 +0800, Yuchao Zhang <ndaugoing@gmail.com> wrote:
>
> [...]
>
> > 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.
>
> This is the same race Hyunwoo reported back in June [1]. You might
> want to have a look at that thread first: Oliver and Marc's view there
> was that the fix is to take the ap_list_lock in
> vgic_v3_fold_lr_state() [2][3], and Hyunwoo posted a draft of that
> [4].
>
> Cheers,
> /fuad
>
> [1] https://lore.kernel.org/r/aiHrGM1f8czcUby4@v4bel
> [2] https://lore.kernel.org/r/aiJi5a3JJ-TbWL-s@kernel.org
> [3] https://lore.kernel.org/r/87a4t99z9n.wl-maz@kernel.org
> [4] https://lore.kernel.org/r/aiXvwGD1hS6vwLEd@v4bel

Re: [PATCH v2] KVM: arm64: vgic: Do not remove in-flight LPIs from AP list on disable

[Fuad Tabba]

Hi Yuchao,

On Fri, 18 Sept 2026 at 12:51, Yuchao Zhang <ndaugoing@gmail.com> wrote:
>
> Hi Fuad,
>
> Thanks a lot for pointing me to that thread! I was not aware of
> Hyunwoo's earlier report and the discussion with Oliver and Marc.
>
> I'll read through the thread and their rationale on the ap_list_lock
> approach. I'm happy to defer to Hyunwoo's effort to avoid duplicate
> work.

I'm not sure you should defer to their effort. It doesn't seem like
Hyunwoo has done any work on this for a while. I just wanted to point
you to the existing discussion.

Cheers,
/fuad

> Thanks again for the pointer!
>
> Best regards,
> Yuchao
>
> Fuad Tabba <fuad.tabba@linux.dev> 于2026年9月18日周五 15:22写道：
> >
> > Hi Yuchao,
> >
> > On Fri, 18 Sep 2026 12:02:14 +0800, Yuchao Zhang <ndaugoing@gmail.com> wrote:
> >
> > [...]
> >
> > > 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.
> >
> > This is the same race Hyunwoo reported back in June [1]. You might
> > want to have a look at that thread first: Oliver and Marc's view there
> > was that the fix is to take the ap_list_lock in
> > vgic_v3_fold_lr_state() [2][3], and Hyunwoo posted a draft of that
> > [4].
> >
> > Cheers,
> > /fuad
> >
> > [1] https://lore.kernel.org/r/aiHrGM1f8czcUby4@v4bel
> > [2] https://lore.kernel.org/r/aiJi5a3JJ-TbWL-s@kernel.org
> > [3] https://lore.kernel.org/r/87a4t99z9n.wl-maz@kernel.org
> > [4] https://lore.kernel.org/r/aiXvwGD1hS6vwLEd@v4bel