[PATCH v3 0/2] KVM: arm64: vgic: Fix racy LPI release and re-registration handling

[PATCH v3 0/2] KVM: arm64: vgic: Fix racy LPI release and re-registration handling

[Carlos López]

Fix a couple of potential issues that could arise from racy LPI release
and re-registration for the same INTID.

The issue fixed in patch 1 can manifest itself through either a
leaked LPI structure, or a prematurely deleted LPI. The issue fixed
in patch 2 could materialize as a spurious -ENOMEM failure when
registering an LPI.

v3:
* Use refcount_dec_and_lock_irqsave() instead of unconditionally
  grabbing the xarray lock in patch 1.
* Add patch 2.

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.

Carlos López (2):
  KVM: arm64: vgic: Fix race between LPI release and re-registration
  KVM: arm64: vgic: Mitigate potential LPI registration failure

 arch/arm64/kvm/vgic/vgic-its.c | 10 +++++++++-
 arch/arm64/kvm/vgic/vgic.c     | 10 ++++++----
 2 files changed, 15 insertions(+), 5 deletions(-)


base-commit: 1ee27dacbe5dc4def481794d899d67b0d4570094
-- 
2.51.0

[PATCH v3 1/2] 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(.., intid)
                                              vgic_try_get_irq_ref(old_irq) == false
                        new IRQ inserted -->  __xa_store(.., intid, ..)
                                              xa_unlock_irqrestore()
  xa_lock_irqsave();
  vgic_release_lpi_locked()
      __xa_erase(.., 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(.., intid)
                                              vgic_try_get_irq_ref(oldirq) == false
                 BUG: old IRQ overwritten --> __xa_store(.., 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>
---
 arch/arm64/kvm/vgic/vgic-its.c | 10 +++++++++-
 arch/arm64/kvm/vgic/vgic.c     | 10 ++++++----
 2 files changed, 15 insertions(+), 5 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..cc09e0c45b46 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -167,12 +167,14 @@ 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);
-	xa_unlock_irqrestore(&dist->lpi_xa, flags);
+	if (refcount_dec_and_lock_irqsave(&irq->refcount,
+					  &dist->lpi_xa.xa_lock, &flags)) {
+		vgic_release_lpi_locked(dist, irq);
+		xa_unlock_irqrestore(&dist->lpi_xa, flags);
+	}
 }
 
 static void vgic_release_deleted_lpis(struct kvm *kvm)
-- 
2.51.0

[PATCH v3 2/2] KVM: arm64: vgic: Mitigate potential LPI registration failure

[Carlos López]

Mitigate a potential failure when inserting a new LPI into the VGIC LPI
xarray.

When vgic_add_lpi() is preparing to register a new LPI, it pre-allocates
an xarray entry using xa_reserve_irq(), so that it can later perform the
insertion under the xarray lock without allocating.

However, since xa_reserve_irq() is called before acquiring such lock,
there is a potential race where xa_reserve_irq() observes a populated
entry, thus not performing the allocation, and another CPU removes that
entry before the xarray lock is grabbed to perform the insertion.

  CPU0 (Adding new LPI)                      CPU1 (Releasing LPI)
  =====================                      ===================
  vgic_add_lpi()
      /* Entry populated, does not allocate */
      xa_reserve_irq(.., intid, ..)
                                            vgic_release_deleted_lpis()
                                                xa_lock_irqsave()
                                                vgic_release_lpi_locked()
                          xarray node freed --> __xa_erase(.., intid)
                                                xa_unlock_irqrestore()
      xa_lock_irqsave()
      xa_load(.., intid) == NULL
      vgic_try_get_irq_ref(NULL) == false
      __xa_store(.., intid, irq, 0) <-- xarray node was freed, gfp=0
                                        cannot allocate, returns -ENOMEM

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.

Mitigate this by passing GFP_NOWAIT to __xa_store(), so that the
allocation can happen under the lock in the rare case that this
condition is hit.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Fixes: 1d6f83f60f79 ("KVM: arm64: vgic: Store LPIs in an xarray")
Signed-off-by: Carlos López <clopez@suse.de>
---
 arch/arm64/kvm/vgic/vgic-its.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 577286069368..ace3e59fff97 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -121,7 +121,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 		 * 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);
+		oldirq = __xa_store(&dist->lpi_xa, intid, irq, GFP_NOWAIT);
 		ret = xa_err(oldirq);
 		if (!ret && oldirq)
 			kfree_rcu(oldirq, rcu);
-- 
2.51.0