[PATCH v2] KVM: arm64: vgic: Fix race between LPI release and re-registration

[PATCH v2] KVM: arm64: vgic: Fix race between LPI release and re-registration

[Carlos López]

Fix a potential race between decrementing an LPI's reference count and
evicting that structure from the LPI xarray.

LPI structures are maintained in the VGIC LPI xarray (dist->lpi_xa).
When the reference count of an LPI structure drops to zero,
vgic_release_lpi_locked() removes the structure from the xarray and
frees it under the xarray lock.

However, the release of an LPI can race with a concurrent LPI
re-registration with the same INTID via vgic_add_lpi() on another CPU,
since the reference count drop and the xarray eviction are not performed
in a single atomic step. This can happen e.g. if the guest issues a
DISCARD while the LPI is still referenced from a vCPU's active-pending
list (ap_list), and the same INTID is re-mapped via MAPTI.

Particularly, vgic_release_lpi_locked() is called from two distinct
paths: direct release via vgic_put_irq(), and deferred release via
vgic_release_deleted_lpis(). During direct release, the issue can result
in deleting a newly registered LPI from the xarray:

  CPU0 (Releasing LPI)                    CPU1 (Adding new LPI)
  ====================                    =====================
  vgic_put_irq()
      __vgic_put_irq()
          refcount_dec_and_test()
                                          vgic_add_lpi()
                                              xa_lock_irqsave(..);
                                              old_irq = xa_load(&dist->lpi_xa, intid);
                                              vgic_try_get_irq_ref(old_irq) == false
                        new IRQ inserted -->  __xa_store(&dist->lpi_xa, intid, ..)
                                              xa_unlock_irqrestore(..);
  xa_lock_irqsave(..);
  vgic_release_lpi_locked()
      __xa_erase(&dist->lpi_xa, irq->intid);   <-- BUG: new IRQ is erased
      kfree_rcu(old_irq)

During the deferred release path, the old IRQ can be leaked:

  CPU0 (Releasing LPI)                    CPU1 (Adding new LPI)
  ====================                    =====================
  vgic_put_irq_norelease()
      __vgic_put_irq()
          refcount_dec_and_test()
      irq->pending_release = true
                                          vgic_add_lpi()
                                              xa_lock_irqsave(..);
                                              old_irq = xa_load(&dist->lpi_xa, intid);
                                              vgic_try_get_irq_ref() == false
                 BUG: old IRQ overwritten --> __xa_store(&dist->lpi_xa, intid, ..)
                                              xa_unlock_irqrestore(..);

  vgic_release_deleted_lpis()
      xa_lock_irqsave(..);
      xa_for_each() { .. } <-- old IRQ with pending_release = true
                               is gone, so it cannot be released

To fix the direct release path, move the reference count drop inside
the xarray lock, making sure that vgic_add_lpi() never encounters the
to-be-released LPI.

To fix the deferred release path, since the refcount drop must happen
under a raw spinlock, the same solution does not work. Instead, update
vgic_add_lpi(), so that if it evicts a non-NULL refcount=0 LPI from the
xarray, it takes on the responsibility of releasing it. If this happens,
vgic_release_deleted_lpis() will iterate the xarray normally and will
simply not find the already released structure.

Reported-by: Claude:claude-opus-4-6
Fixes: 3a08a6ca7c37 ("KVM: arm64: vgic-v3: Use bare refcount for VGIC LPIs")
Fixes: d54594accf73 ("KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks")
Signed-off-by: Carlos López <clopez@suse.de>
---
v2:
* Address Sashiko's review. Fix the direct release path by decrementing the
  refcount under the xarray spinlock, preventing a UAF that would have been
  introduced in v1.
---
 arch/arm64/kvm/vgic/vgic-its.c | 10 +++++++++-
 arch/arm64/kvm/vgic/vgic.c     |  5 +++--
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 67d107e9a77d..577286069368 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -116,7 +116,15 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 		kfree(irq);
 		irq = oldirq;
 	} else {
-		ret = xa_err(__xa_store(&dist->lpi_xa, intid, irq, 0));
+		/*
+		 * The entry is either empty or contains a dead LPI (refcount=0)
+		 * from the deferred release path, pending cleanup by
+		 * vgic_release_deleted_lpis(). Evict and free it if present.
+		 */
+		oldirq = __xa_store(&dist->lpi_xa, intid, irq, 0);
+		ret = xa_err(oldirq);
+		if (!ret && oldirq)
+			kfree_rcu(oldirq, rcu);
 	}
 
 	xa_unlock_irqrestore(&dist->lpi_xa, flags);
diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index 5a4768d8cd4f..c32e6c9777e5 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -167,11 +167,12 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 		guard(spinlock_irqsave)(&dist->lpi_xa.xa_lock);
 	}
 
-	if (!__vgic_put_irq(kvm, irq))
+	if (!irq_is_lpi(kvm, irq->intid))
 		return;
 
 	xa_lock_irqsave(&dist->lpi_xa, flags);
-	vgic_release_lpi_locked(dist, irq);
+	if (refcount_dec_and_test(&irq->refcount))
+		vgic_release_lpi_locked(dist, irq);
 	xa_unlock_irqrestore(&dist->lpi_xa, flags);
 }
 

base-commit: 1ee27dacbe5dc4def481794d899d67b0d4570094
-- 
2.51.0

Re: [PATCH v2] KVM: arm64: vgic: Fix race between LPI release and re-registration

[Oliver Upton]

Hi Carlos,

Thanks for reporting this ugly bug.

On Fri, Jul 03, 2026 at 04:15:08AM +0200, Carlos López wrote:
> To fix the direct release path, move the reference count drop inside
> the xarray lock, making sure that vgic_add_lpi() never encounters the
> to-be-released LPI.

As Sashiko pointed out, this is going to massively regress performance
of LPI injection. I don't think this is going to be a viable option.

> To fix the deferred release path, since the refcount drop must happen
> under a raw spinlock, the same solution does not work. Instead, update
> vgic_add_lpi(), so that if it evicts a non-NULL refcount=0 LPI from the
> xarray, it takes on the responsibility of releasing it. If this happens,
> vgic_release_deleted_lpis() will iterate the xarray normally and will
> simply not find the already released structure.
> 
> Reported-by: Claude:claude-opus-4-6
> Fixes: 3a08a6ca7c37 ("KVM: arm64: vgic-v3: Use bare refcount for VGIC LPIs")
> Fixes: d54594accf73 ("KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks")
> Signed-off-by: Carlos López <clopez@suse.de>
> ---
> v2:
> * Address Sashiko's review. Fix the direct release path by decrementing the
>   refcount under the xarray spinlock, preventing a UAF that would have been
>   introduced in v1.

So I actually agree with your approach in v1, vgic_release_lpi_locked()
should do an __xa_cmpxchg() to only erase if the to-be-deleted IRQ that
it owns remains in the xarray.

I believe the UAF could've been avoided by unconditionally calling
kfree_rcu() in vgic_release_lpi_locked() and not attempting to cleanup
dead LPIs in vgic_add_lpi(). IOW, whoever takes the refcount of an LPI
to 0 always has the responsibility of freeing it.

Maybe below would be enough?

Thanks,
Oliver

diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index 5a4768d8cd4f..4c79e1096af4 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -132,7 +132,14 @@ struct vgic_irq *vgic_get_vcpu_irq(struct kvm_vcpu *vcpu, u32 intid)
 static void vgic_release_lpi_locked(struct vgic_dist *dist, struct vgic_irq *irq)
 {
 	lockdep_assert_held(&dist->lpi_xa.xa_lock);
-	__xa_erase(&dist->lpi_xa, irq->intid);
+
+	/*
+	 * Another LPI could've been inserted prior to taking the xa_lock, as
+	 * vgic_add_lpi() can only take a reference on a pre-existing LPI if
+	 * the refcount is nonzero. While freeing the object is always done here,
+	 * only delete the entry @INTID if it is this IRQ.
+	 */
+	__xa_cmpxchg(&dist->lpi_xa, irq->intid, irq, NULL, 0);
 	kfree_rcu(irq, rcu);
 }
 

Re: [PATCH v2] KVM: arm64: vgic: Fix race between LPI release and re-registration

[Carlos López]

Hi,

I was about to reply to Sashiko, I'd rather talk to a human :)

On 7/3/26 10:44 AM, Oliver Upton wrote:
> Hi Carlos,
> 
> Thanks for reporting this ugly bug.
> 
> On Fri, Jul 03, 2026 at 04:15:08AM +0200, Carlos López wrote:
>> To fix the direct release path, move the reference count drop inside
>> the xarray lock, making sure that vgic_add_lpi() never encounters the
>> to-be-released LPI.
> 
> As Sashiko pointed out, this is going to massively regress performance
> of LPI injection. I don't think this is going to be a viable option.

I think we can just use refcount_dec_and_lock_irqsave(), no? Then we
grab the lock only if the refcount drops to 0.

As for the other issue (spurious -ENOMEM on __xa_store()), it's a
preexisting issue, but should be fixed by just passing GFP_NOWAIT to
__xa_store(). I can add another patch in v3 for this.

>> To fix the deferred release path, since the refcount drop must happen
>> under a raw spinlock, the same solution does not work. Instead, update
>> vgic_add_lpi(), so that if it evicts a non-NULL refcount=0 LPI from the
>> xarray, it takes on the responsibility of releasing it. If this happens,
>> vgic_release_deleted_lpis() will iterate the xarray normally and will
>> simply not find the already released structure.
>>
>> Reported-by: Claude:claude-opus-4-6
>> Fixes: 3a08a6ca7c37 ("KVM: arm64: vgic-v3: Use bare refcount for VGIC LPIs")
>> Fixes: d54594accf73 ("KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks")
>> Signed-off-by: Carlos López <clopez@suse.de>
>> ---
>> v2:
>> * Address Sashiko's review. Fix the direct release path by decrementing the
>>   refcount under the xarray spinlock, preventing a UAF that would have been
>>   introduced in v1.
> 
> So I actually agree with your approach in v1, vgic_release_lpi_locked()
> should do an __xa_cmpxchg() to only erase if the to-be-deleted IRQ that
> it owns remains in the xarray.
> 
> I believe the UAF could've been avoided by unconditionally calling
> kfree_rcu() in vgic_release_lpi_locked() and not attempting to cleanup
> dead LPIs in vgic_add_lpi(). IOW, whoever takes the refcount of an LPI
> to 0 always has the responsibility of freeing it.


I think this would solve the direct release path, but not the deferred
path. If vgic_add_lpi() does not perform any cleanup, and encounters an
IRQ that was vgic_put_irq_norelease()-ed before
vgic_release_deleted_lpis() grabs the xarray lock then the struct is
overwritten without being released.

> Maybe below would be enough?
> 
> Thanks,
> Oliver
> 
> diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> index 5a4768d8cd4f..4c79e1096af4 100644
> --- a/arch/arm64/kvm/vgic/vgic.c
> +++ b/arch/arm64/kvm/vgic/vgic.c
> @@ -132,7 +132,14 @@ struct vgic_irq *vgic_get_vcpu_irq(struct kvm_vcpu *vcpu, u32 intid)
>  static void vgic_release_lpi_locked(struct vgic_dist *dist, struct vgic_irq *irq)
>  {
>  	lockdep_assert_held(&dist->lpi_xa.xa_lock);
> -	__xa_erase(&dist->lpi_xa, irq->intid);
> +
> +	/*
> +	 * Another LPI could've been inserted prior to taking the xa_lock, as
> +	 * vgic_add_lpi() can only take a reference on a pre-existing LPI if
> +	 * the refcount is nonzero. While freeing the object is always done here,
> +	 * only delete the entry @INTID if it is this IRQ.
> +	 */
> +	__xa_cmpxchg(&dist->lpi_xa, irq->intid, irq, NULL, 0);
>  	kfree_rcu(irq, rcu);
>  }
>