[PATCH 0/5] KVM: arm64: vgic-v3: Fix yet another lock ordering turd

[PATCH 0/5] KVM: arm64: vgic-v3: Fix yet another lock ordering turd

[Oliver Upton]

syzkaller found yet another locking bug in the VGIC [*], this time due
to nesting a 'plain' spinlock (xa_lock) inside of a raw spinlock
(ap_list_lock). Given the way we do refcounts on LPIs it is possible
for this exact sort of issue to crop up where the last reference may
be dropped in unexpected places.

Small series to fix the issue by deferring xarray modifications outside
of the ap_list_lock critical section along with some slight lockdep
hinting to make these rare bugs a bit more obvious.

Applies to 6.17-rc4.

Oliver Upton (5):
  KVM: arm64: vgic-v3: Use bare refcount for VGIC LPIs
  KVM: arm64: Spin off release helper from vgic_put_irq()
  KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks
  KVM: arm64: vgic-v3: Don't require IRQs be disabled for LPI xarray
    lock
  KVM: arm64: vgic-v3: Indicate vgic_put_irq() may take LPI xarray lock

 arch/arm64/kvm/vgic/vgic-debug.c |  2 +-
 arch/arm64/kvm/vgic/vgic-init.c  |  6 +--
 arch/arm64/kvm/vgic/vgic-its.c   | 15 +++---
 arch/arm64/kvm/vgic/vgic-v4.c    |  2 +-
 arch/arm64/kvm/vgic/vgic.c       | 78 +++++++++++++++++++++++---------
 arch/arm64/kvm/vgic/vgic.h       |  8 ++--
 include/kvm/arm_vgic.h           | 10 ++--
 7 files changed, 80 insertions(+), 41 deletions(-)


base-commit: b320789d6883cc00ac78ce83bccbfe7ed58afcf0
-- 
2.39.5

[PATCH 1/5] KVM: arm64: vgic-v3: Use bare refcount for VGIC LPIs

[Oliver Upton]

KVM's use of krefs to manage LPIs isn't adding much, especially
considering vgic_irq_release() is a noop due to the lack of sufficient
context.

Switch to using a regular refcount in anticipation of adding a
meaningful release concept for LPIs.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/vgic/vgic-debug.c |  2 +-
 arch/arm64/kvm/vgic/vgic-init.c  |  4 ++--
 arch/arm64/kvm/vgic/vgic-its.c   |  8 ++++----
 arch/arm64/kvm/vgic/vgic-v4.c    |  2 +-
 arch/arm64/kvm/vgic/vgic.c       | 17 ++++-------------
 arch/arm64/kvm/vgic/vgic.h       |  8 ++++----
 include/kvm/arm_vgic.h           |  4 ++--
 7 files changed, 18 insertions(+), 27 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-debug.c b/arch/arm64/kvm/vgic/vgic-debug.c
index 2684f273d9e1..4c1209261b65 100644
--- a/arch/arm64/kvm/vgic/vgic-debug.c
+++ b/arch/arm64/kvm/vgic/vgic-debug.c
@@ -69,7 +69,7 @@ static int iter_mark_lpis(struct kvm *kvm)
 	int nr_lpis = 0;
 
 	xa_for_each(&dist->lpi_xa, intid, irq) {
-		if (!vgic_try_get_irq_kref(irq))
+		if (!vgic_try_get_irq_ref(irq))
 			continue;
 
 		xa_set_mark(&dist->lpi_xa, intid, LPI_XA_MARK_DEBUG_ITER);
diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index 1e680ad6e863..4c777136ea5f 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -208,7 +208,7 @@ static int kvm_vgic_dist_init(struct kvm *kvm, unsigned int nr_spis)
 		raw_spin_lock_init(&irq->irq_lock);
 		irq->vcpu = NULL;
 		irq->target_vcpu = vcpu0;
-		kref_init(&irq->refcount);
+		refcount_set(&irq->refcount, 0);
 		switch (dist->vgic_model) {
 		case KVM_DEV_TYPE_ARM_VGIC_V2:
 			irq->targets = 0;
@@ -277,7 +277,7 @@ static int vgic_allocate_private_irqs_locked(struct kvm_vcpu *vcpu, u32 type)
 		irq->intid = i;
 		irq->vcpu = NULL;
 		irq->target_vcpu = vcpu;
-		kref_init(&irq->refcount);
+		refcount_set(&irq->refcount, 0);
 		if (vgic_irq_is_sgi(i)) {
 			/* SGIs */
 			irq->enabled = 1;
diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index 7368c13f16b7..f162206adb48 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -99,7 +99,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 	raw_spin_lock_init(&irq->irq_lock);
 
 	irq->config = VGIC_CONFIG_EDGE;
-	kref_init(&irq->refcount);
+	refcount_set(&irq->refcount, 1);
 	irq->intid = intid;
 	irq->target_vcpu = vcpu;
 	irq->group = 1;
@@ -111,7 +111,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 	 * check that we don't add a second list entry with the same LPI.
 	 */
 	oldirq = xa_load(&dist->lpi_xa, intid);
-	if (vgic_try_get_irq_kref(oldirq)) {
+	if (vgic_try_get_irq_ref(oldirq)) {
 		/* Someone was faster with adding this LPI, lets use that. */
 		kfree(irq);
 		irq = oldirq;
@@ -547,7 +547,7 @@ static struct vgic_irq *vgic_its_check_cache(struct kvm *kvm, phys_addr_t db,
 	rcu_read_lock();
 
 	irq = xa_load(&its->translation_cache, cache_key);
-	if (!vgic_try_get_irq_kref(irq))
+	if (!vgic_try_get_irq_ref(irq))
 		irq = NULL;
 
 	rcu_read_unlock();
@@ -571,7 +571,7 @@ static void vgic_its_cache_translation(struct kvm *kvm, struct vgic_its *its,
 	 * its_lock, as the ITE (and the reference it holds) cannot be freed.
 	 */
 	lockdep_assert_held(&its->its_lock);
-	vgic_get_irq_kref(irq);
+	vgic_get_irq_ref(irq);
 
 	old = xa_store(&its->translation_cache, cache_key, irq, GFP_KERNEL_ACCOUNT);
 
diff --git a/arch/arm64/kvm/vgic/vgic-v4.c b/arch/arm64/kvm/vgic/vgic-v4.c
index 4d9343d2b0b1..548aec9d5a72 100644
--- a/arch/arm64/kvm/vgic/vgic-v4.c
+++ b/arch/arm64/kvm/vgic/vgic-v4.c
@@ -518,7 +518,7 @@ static struct vgic_irq *__vgic_host_irq_get_vlpi(struct kvm *kvm, int host_irq)
 		if (!irq->hw || irq->host_irq != host_irq)
 			continue;
 
-		if (!vgic_try_get_irq_kref(irq))
+		if (!vgic_try_get_irq_ref(irq))
 			return NULL;
 
 		return irq;
diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index f5148b38120a..a1d6fab895c4 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -71,7 +71,7 @@ static struct vgic_irq *vgic_get_lpi(struct kvm *kvm, u32 intid)
 	rcu_read_lock();
 
 	irq = xa_load(&dist->lpi_xa, intid);
-	if (!vgic_try_get_irq_kref(irq))
+	if (!vgic_try_get_irq_ref(irq))
 		irq = NULL;
 
 	rcu_read_unlock();
@@ -114,15 +114,6 @@ struct vgic_irq *vgic_get_vcpu_irq(struct kvm_vcpu *vcpu, u32 intid)
 	return vgic_get_irq(vcpu->kvm, intid);
 }
 
-/*
- * We can't do anything in here, because we lack the kvm pointer to
- * lock and remove the item from the lpi_list. So we keep this function
- * empty and use the return value of kref_put() to trigger the freeing.
- */
-static void vgic_irq_release(struct kref *ref)
-{
-}
-
 void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
@@ -131,7 +122,7 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 	if (irq->intid < VGIC_MIN_LPI)
 		return;
 
-	if (!kref_put(&irq->refcount, vgic_irq_release))
+	if (!refcount_dec_and_test(&irq->refcount))
 		return;
 
 	xa_lock_irqsave(&dist->lpi_xa, flags);
@@ -399,7 +390,7 @@ bool vgic_queue_irq_unlock(struct kvm *kvm, struct vgic_irq *irq,
 	 * now in the ap_list. This is safe as the caller must already hold a
 	 * reference on the irq.
 	 */
-	vgic_get_irq_kref(irq);
+	vgic_get_irq_ref(irq);
 	list_add_tail(&irq->ap_list, &vcpu->arch.vgic_cpu.ap_list_head);
 	irq->vcpu = vcpu;
 
@@ -657,7 +648,7 @@ static void vgic_prune_ap_list(struct kvm_vcpu *vcpu)
 
 			/*
 			 * This vgic_put_irq call matches the
-			 * vgic_get_irq_kref in vgic_queue_irq_unlock,
+			 * vgic_get_irq_ref in vgic_queue_irq_unlock,
 			 * where we added the LPI to the ap_list. As
 			 * we remove the irq from the list, we drop
 			 * also drop the refcount.
diff --git a/arch/arm64/kvm/vgic/vgic.h b/arch/arm64/kvm/vgic/vgic.h
index de1c1d3261c3..ac5f9c5d2b98 100644
--- a/arch/arm64/kvm/vgic/vgic.h
+++ b/arch/arm64/kvm/vgic/vgic.h
@@ -267,7 +267,7 @@ void vgic_v2_put(struct kvm_vcpu *vcpu);
 void vgic_v2_save_state(struct kvm_vcpu *vcpu);
 void vgic_v2_restore_state(struct kvm_vcpu *vcpu);
 
-static inline bool vgic_try_get_irq_kref(struct vgic_irq *irq)
+static inline bool vgic_try_get_irq_ref(struct vgic_irq *irq)
 {
 	if (!irq)
 		return false;
@@ -275,12 +275,12 @@ static inline bool vgic_try_get_irq_kref(struct vgic_irq *irq)
 	if (irq->intid < VGIC_MIN_LPI)
 		return true;
 
-	return kref_get_unless_zero(&irq->refcount);
+	return refcount_inc_not_zero(&irq->refcount);
 }
 
-static inline void vgic_get_irq_kref(struct vgic_irq *irq)
+static inline void vgic_get_irq_ref(struct vgic_irq *irq)
 {
-	WARN_ON_ONCE(!vgic_try_get_irq_kref(irq));
+	WARN_ON_ONCE(!vgic_try_get_irq_ref(irq));
 }
 
 void vgic_v3_fold_lr_state(struct kvm_vcpu *vcpu);
diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
index 404883c7af6e..ed0e96031a65 100644
--- a/include/kvm/arm_vgic.h
+++ b/include/kvm/arm_vgic.h
@@ -8,8 +8,8 @@
 #include <linux/bits.h>
 #include <linux/kvm.h>
 #include <linux/irqreturn.h>
-#include <linux/kref.h>
 #include <linux/mutex.h>
+#include <linux/refcount.h>
 #include <linux/spinlock.h>
 #include <linux/static_key.h>
 #include <linux/types.h>
@@ -142,7 +142,7 @@ struct vgic_irq {
 	bool active;			/* not used for LPIs */
 	bool enabled;
 	bool hw;			/* Tied to HW IRQ */
-	struct kref refcount;		/* Used for LPIs */
+	refcount_t refcount;		/* Used for LPIs */
 	u32 hwintid;			/* HW INTID number */
 	unsigned int host_irq;		/* linux irq corresponding to hwintid */
 	union {
-- 
2.39.5

[PATCH 2/5] KVM: arm64: Spin off release helper from vgic_put_irq()

[Oliver Upton]

Spin off the release implementation from vgic_put_irq() to prepare for a
more involved fix for lock ordering such that it may be unnested from
raw spinlocks. This has the minor functional change of doing call_rcu()
behind the xa_lock although it shouldn't be consequential.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/vgic/vgic.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index a1d6fab895c4..ec4d70936a5b 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -114,22 +114,32 @@ struct vgic_irq *vgic_get_vcpu_irq(struct kvm_vcpu *vcpu, u32 intid)
 	return vgic_get_irq(vcpu->kvm, 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);
+	kfree_rcu(irq, rcu);
+}
+
+static __must_check bool __vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
+{
+	if (irq->intid < VGIC_MIN_LPI)
+		return false;
+
+	return refcount_dec_and_test(&irq->refcount);
+}
+
 void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
 	unsigned long flags;
 
-	if (irq->intid < VGIC_MIN_LPI)
-		return;
-
-	if (!refcount_dec_and_test(&irq->refcount))
+	if (!__vgic_put_irq(kvm, irq))
 		return;
 
 	xa_lock_irqsave(&dist->lpi_xa, flags);
-	__xa_erase(&dist->lpi_xa, irq->intid);
+	vgic_release_lpi_locked(dist, irq);
 	xa_unlock_irqrestore(&dist->lpi_xa, flags);
-
-	kfree_rcu(irq, rcu);
 }
 
 void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
-- 
2.39.5

[PATCH 3/5] KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks

[Oliver Upton]

syzkaller has caught us red-handed once more, this time nesting regular
spinlocks behind raw spinlocks:

  =============================
  [ BUG: Invalid wait context ]
  6.16.0-rc3-syzkaller-g7b8346bd9fce #0 Not tainted
  -----------------------------
  syz.0.29/3743 is trying to lock:
  a3ff80008e2e9e18 (&xa->xa_lock#20){....}-{3:3}, at: vgic_put_irq+0xb4/0x190 arch/arm64/kvm/vgic/vgic.c:137
  other info that might help us debug this:
  context-{5:5}
  3 locks held by syz.0.29/3743:
   #0: a3ff80008e2e90a8 (&kvm->slots_lock){+.+.}-{4:4}, at: kvm_vgic_destroy+0x50/0x624 arch/arm64/kvm/vgic/vgic-init.c:499
   #1: a3ff80008e2e9fa0 (&kvm->arch.config_lock){+.+.}-{4:4}, at: kvm_vgic_destroy+0x5c/0x624 arch/arm64/kvm/vgic/vgic-init.c:500
   #2: 58f0000021be1428 (&vgic_cpu->ap_list_lock){....}-{2:2}, at: vgic_flush_pending_lpis+0x3c/0x31c arch/arm64/kvm/vgic/vgic.c:150
  stack backtrace:
  CPU: 0 UID: 0 PID: 3743 Comm: syz.0.29 Not tainted 6.16.0-rc3-syzkaller-g7b8346bd9fce #0 PREEMPT
  Hardware name: linux,dummy-virt (DT)
  Call trace:
   show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:466 (C)
   __dump_stack+0x30/0x40 lib/dump_stack.c:94
   dump_stack_lvl+0xd8/0x12c lib/dump_stack.c:120
   dump_stack+0x1c/0x28 lib/dump_stack.c:129
   print_lock_invalid_wait_context kernel/locking/lockdep.c:4833 [inline]
   check_wait_context kernel/locking/lockdep.c:4905 [inline]
   __lock_acquire+0x978/0x299c kernel/locking/lockdep.c:5190
   lock_acquire+0x14c/0x2e0 kernel/locking/lockdep.c:5871
   __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
   _raw_spin_lock_irqsave+0x5c/0x7c kernel/locking/spinlock.c:162
   vgic_put_irq+0xb4/0x190 arch/arm64/kvm/vgic/vgic.c:137
   vgic_flush_pending_lpis+0x24c/0x31c arch/arm64/kvm/vgic/vgic.c:158
   __kvm_vgic_vcpu_destroy+0x44/0x500 arch/arm64/kvm/vgic/vgic-init.c:455
   kvm_vgic_destroy+0x100/0x624 arch/arm64/kvm/vgic/vgic-init.c:505
   kvm_arch_destroy_vm+0x80/0x138 arch/arm64/kvm/arm.c:244
   kvm_destroy_vm virt/kvm/kvm_main.c:1308 [inline]
   kvm_put_kvm+0x800/0xff8 virt/kvm/kvm_main.c:1344
   kvm_vm_release+0x58/0x78 virt/kvm/kvm_main.c:1367
   __fput+0x4ac/0x980 fs/file_table.c:465
   ____fput+0x20/0x58 fs/file_table.c:493
   task_work_run+0x1bc/0x254 kernel/task_work.c:227
   resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
   do_notify_resume+0x1b4/0x270 arch/arm64/kernel/entry-common.c:151
   exit_to_user_mode_prepare arch/arm64/kernel/entry-common.c:169 [inline]
   exit_to_user_mode arch/arm64/kernel/entry-common.c:178 [inline]
   el0_svc+0xb4/0x160 arch/arm64/kernel/entry-common.c:768
   el0t_64_sync_handler+0x78/0x108 arch/arm64/kernel/entry-common.c:786
   el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:600

This is of course no good, but is at odds with how LPI refcounts are
managed. Solve the locking mess by deferring the release of unreferenced
LPIs after the ap_list_lock is released. Mark these to-be-released LPIs
specially to avoid racing with vgic_put_irq() and causing a double-free.

Since references can only be taken on LPIs with a nonzero refcount,
extending the lifetime of freed LPIs is still safe.

Reported-by: syzbot+cef594105ac7e60c6d93@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/kvmarm/68acd0d9.a00a0220.33401d.048b.GAE@google.com/
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/vgic/vgic.c | 41 ++++++++++++++++++++++++++++++++++----
 include/kvm/arm_vgic.h     |  6 +++++-
 2 files changed, 42 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index ec4d70936a5b..480391a0dcad 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -28,8 +28,8 @@ struct vgic_global kvm_vgic_global_state __ro_after_init = {
  *     kvm->arch.config_lock (mutex)
  *       its->cmd_lock (mutex)
  *         its->its_lock (mutex)
- *           vgic_cpu->ap_list_lock		must be taken with IRQs disabled
- *             vgic_dist->lpi_xa.xa_lock	must be taken with IRQs disabled
+ *           vgic_dist->lpi_xa.xa_lock		must be taken with IRQs disabled
+ *             vgic_cpu->ap_list_lock		must be taken with IRQs disabled
  *               vgic_irq->irq_lock		must be taken with IRQs disabled
  *
  * As the ap_list_lock might be taken from the timer interrupt handler,
@@ -129,6 +129,15 @@ static __must_check bool __vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 	return refcount_dec_and_test(&irq->refcount);
 }
 
+static __must_check bool vgic_put_irq_norelease(struct kvm *kvm, struct vgic_irq *irq)
+{
+	if (!__vgic_put_irq(kvm, irq))
+		return false;
+
+	irq->pending_release = true;
+	return true;
+}
+
 void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
@@ -142,10 +151,27 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 	xa_unlock_irqrestore(&dist->lpi_xa, flags);
 }
 
+static void vgic_release_deleted_lpis(struct kvm *kvm)
+{
+	struct vgic_dist *dist = &kvm->arch.vgic;
+	unsigned long flags, intid;
+	struct vgic_irq *irq;
+
+	xa_lock_irqsave(&dist->lpi_xa, flags);
+
+	xa_for_each(&dist->lpi_xa, intid, irq) {
+		if (irq->pending_release)
+			vgic_release_lpi_locked(dist, irq);
+	}
+
+	xa_unlock_irqrestore(&dist->lpi_xa, flags);
+}
+
 void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
 {
 	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
 	struct vgic_irq *irq, *tmp;
+	bool deleted = false;
 	unsigned long flags;
 
 	raw_spin_lock_irqsave(&vgic_cpu->ap_list_lock, flags);
@@ -156,11 +182,14 @@ void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
 			list_del(&irq->ap_list);
 			irq->vcpu = NULL;
 			raw_spin_unlock(&irq->irq_lock);
-			vgic_put_irq(vcpu->kvm, irq);
+			deleted |= vgic_put_irq_norelease(vcpu->kvm, irq);
 		}
 	}
 
 	raw_spin_unlock_irqrestore(&vgic_cpu->ap_list_lock, flags);
+
+	if (deleted)
+		vgic_release_deleted_lpis(vcpu->kvm);
 }
 
 void vgic_irq_set_phys_pending(struct vgic_irq *irq, bool pending)
@@ -631,6 +660,7 @@ static void vgic_prune_ap_list(struct kvm_vcpu *vcpu)
 {
 	struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
 	struct vgic_irq *irq, *tmp;
+	bool deleted_lpis = false;
 
 	DEBUG_SPINLOCK_BUG_ON(!irqs_disabled());
 
@@ -663,7 +693,7 @@ static void vgic_prune_ap_list(struct kvm_vcpu *vcpu)
 			 * we remove the irq from the list, we drop
 			 * also drop the refcount.
 			 */
-			vgic_put_irq(vcpu->kvm, irq);
+			deleted_lpis |= vgic_put_irq_norelease(vcpu->kvm, irq);
 			continue;
 		}
 
@@ -726,6 +756,9 @@ static void vgic_prune_ap_list(struct kvm_vcpu *vcpu)
 	}
 
 	raw_spin_unlock(&vgic_cpu->ap_list_lock);
+
+	if (unlikely(deleted_lpis))
+		vgic_release_deleted_lpis(vcpu->kvm);
 }
 
 static inline void vgic_fold_lr_state(struct kvm_vcpu *vcpu)
diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
index ed0e96031a65..af224db3cf72 100644
--- a/include/kvm/arm_vgic.h
+++ b/include/kvm/arm_vgic.h
@@ -139,7 +139,11 @@ struct vgic_irq {
 	bool pending_latch;		/* The pending latch state used to calculate
 					 * the pending state for both level
 					 * and edge triggered IRQs. */
-	bool active;			/* not used for LPIs */
+	union {
+		bool active;		/* not used for LPIs */
+		bool pending_release;	/* LPI pending a release */
+	};
+
 	bool enabled;
 	bool hw;			/* Tied to HW IRQ */
 	refcount_t refcount;		/* Used for LPIs */
-- 
2.39.5

Re: [PATCH 3/5] KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks

[Marc Zyngier]

On Thu, 04 Sep 2025 07:23:46 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
> index ed0e96031a65..af224db3cf72 100644
> --- a/include/kvm/arm_vgic.h
> +++ b/include/kvm/arm_vgic.h
> @@ -139,7 +139,11 @@ struct vgic_irq {
>  	bool pending_latch;		/* The pending latch state used to calculate
>  					 * the pending state for both level
>  					 * and edge triggered IRQs. */
> -	bool active;			/* not used for LPIs */
> +	union {
> +		bool active;		/* not used for LPIs */
> +		bool pending_release;	/* LPI pending a release */
> +	};
> +

Err... no. Please don't do that. An activated LPI that hasn't been
EOI'd yet does have the active state in the LR (yes, the original
comment is totally broken, please remove it).

Imagine a case where you'd preempt the guest between the read of IAR
and the write to EOI: pending_release is now set, and that could
result in some funky stuff...

Just add it as a separate field, I don't think anyone will cry over
that.

	M.

-- 
Jazz isn't dead. It just smells funny.

Re: [PATCH 3/5] KVM: arm64: vgic-v3: Erase LPIs from xarray outside of raw spinlocks

[Oliver Upton]

On Fri, Sep 05, 2025 at 08:44:38AM +0100, Marc Zyngier wrote:
> On Thu, 04 Sep 2025 07:23:46 +0100,
> Oliver Upton <oliver.upton@linux.dev> wrote:
> > 
> > diff --git a/include/kvm/arm_vgic.h b/include/kvm/arm_vgic.h
> > index ed0e96031a65..af224db3cf72 100644
> > --- a/include/kvm/arm_vgic.h
> > +++ b/include/kvm/arm_vgic.h
> > @@ -139,7 +139,11 @@ struct vgic_irq {
> >  	bool pending_latch;		/* The pending latch state used to calculate
> >  					 * the pending state for both level
> >  					 * and edge triggered IRQs. */
> > -	bool active;			/* not used for LPIs */
> > +	union {
> > +		bool active;		/* not used for LPIs */
> > +		bool pending_release;	/* LPI pending a release */
> > +	};
> > +
> 
> Err... no. Please don't do that. An activated LPI that hasn't been
> EOI'd yet does have the active state in the LR (yes, the original
> comment is totally broken, please remove it).

Yeah, this obviously wasn't thought about too carefully. I'll squash in
a fix unless there's something more ugly that needs addressing.

Thanks,
Oliver

[PATCH 4/5] KVM: arm64: vgic-v3: Don't require IRQs be disabled for LPI xarray lock

[Oliver Upton]

Now that the LPI xarray is only read with IRQs disabled, relax the
locking on the xarray to a non-IRQ disabling spinlock.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/vgic/vgic-init.c |  2 +-
 arch/arm64/kvm/vgic/vgic-its.c  |  7 +++----
 arch/arm64/kvm/vgic/vgic.c      | 13 ++++++-------
 3 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
index 4c777136ea5f..4c3c0d82e476 100644
--- a/arch/arm64/kvm/vgic/vgic-init.c
+++ b/arch/arm64/kvm/vgic/vgic-init.c
@@ -53,7 +53,7 @@ void kvm_vgic_early_init(struct kvm *kvm)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
 
-	xa_init_flags(&dist->lpi_xa, XA_FLAGS_LOCK_IRQ);
+	xa_init(&dist->lpi_xa);
 }
 
 /* CREATION */
diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
index f162206adb48..ce3e3ed3f29f 100644
--- a/arch/arm64/kvm/vgic/vgic-its.c
+++ b/arch/arm64/kvm/vgic/vgic-its.c
@@ -78,7 +78,6 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
 	struct vgic_irq *irq = vgic_get_irq(kvm, intid), *oldirq;
-	unsigned long flags;
 	int ret;
 
 	/* In this case there is no put, since we keep the reference. */
@@ -89,7 +88,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 	if (!irq)
 		return ERR_PTR(-ENOMEM);
 
-	ret = xa_reserve_irq(&dist->lpi_xa, intid, GFP_KERNEL_ACCOUNT);
+	ret = xa_reserve(&dist->lpi_xa, intid, GFP_KERNEL_ACCOUNT);
 	if (ret) {
 		kfree(irq);
 		return ERR_PTR(ret);
@@ -104,7 +103,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 	irq->target_vcpu = vcpu;
 	irq->group = 1;
 
-	xa_lock_irqsave(&dist->lpi_xa, flags);
+	xa_lock(&dist->lpi_xa);
 
 	/*
 	 * There could be a race with another vgic_add_lpi(), so we need to
@@ -126,7 +125,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
 	}
 
 out_unlock:
-	xa_unlock_irqrestore(&dist->lpi_xa, flags);
+	xa_unlock(&dist->lpi_xa);
 
 	if (ret)
 		return ERR_PTR(ret);
diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index 480391a0dcad..a21b482844ce 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -28,7 +28,7 @@ struct vgic_global kvm_vgic_global_state __ro_after_init = {
  *     kvm->arch.config_lock (mutex)
  *       its->cmd_lock (mutex)
  *         its->its_lock (mutex)
- *           vgic_dist->lpi_xa.xa_lock		must be taken with IRQs disabled
+ *           vgic_dist->lpi_xa.xa_lock
  *             vgic_cpu->ap_list_lock		must be taken with IRQs disabled
  *               vgic_irq->irq_lock		must be taken with IRQs disabled
  *
@@ -141,30 +141,29 @@ static __must_check bool vgic_put_irq_norelease(struct kvm *kvm, struct vgic_irq
 void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
-	unsigned long flags;
 
 	if (!__vgic_put_irq(kvm, irq))
 		return;
 
-	xa_lock_irqsave(&dist->lpi_xa, flags);
+	xa_lock(&dist->lpi_xa);
 	vgic_release_lpi_locked(dist, irq);
-	xa_unlock_irqrestore(&dist->lpi_xa, flags);
+	xa_unlock(&dist->lpi_xa);
 }
 
 static void vgic_release_deleted_lpis(struct kvm *kvm)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
-	unsigned long flags, intid;
+	unsigned long intid;
 	struct vgic_irq *irq;
 
-	xa_lock_irqsave(&dist->lpi_xa, flags);
+	xa_lock(&dist->lpi_xa);
 
 	xa_for_each(&dist->lpi_xa, intid, irq) {
 		if (irq->pending_release)
 			vgic_release_lpi_locked(dist, irq);
 	}
 
-	xa_unlock_irqrestore(&dist->lpi_xa, flags);
+	xa_unlock(&dist->lpi_xa);
 }
 
 void vgic_flush_pending_lpis(struct kvm_vcpu *vcpu)
-- 
2.39.5

Re: [PATCH 4/5] KVM: arm64: vgic-v3: Don't require IRQs be disabled for LPI xarray lock

[Marc Zyngier]

On Thu, 04 Sep 2025 07:23:47 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> Now that the LPI xarray is only read with IRQs disabled, relax the
> locking on the xarray to a non-IRQ disabling spinlock.
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/vgic/vgic-init.c |  2 +-
>  arch/arm64/kvm/vgic/vgic-its.c  |  7 +++----
>  arch/arm64/kvm/vgic/vgic.c      | 13 ++++++-------
>  3 files changed, 10 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c
> index 4c777136ea5f..4c3c0d82e476 100644
> --- a/arch/arm64/kvm/vgic/vgic-init.c
> +++ b/arch/arm64/kvm/vgic/vgic-init.c
> @@ -53,7 +53,7 @@ void kvm_vgic_early_init(struct kvm *kvm)
>  {
>  	struct vgic_dist *dist = &kvm->arch.vgic;
>  
> -	xa_init_flags(&dist->lpi_xa, XA_FLAGS_LOCK_IRQ);
> +	xa_init(&dist->lpi_xa);
>  }
>  
>  /* CREATION */
> diff --git a/arch/arm64/kvm/vgic/vgic-its.c b/arch/arm64/kvm/vgic/vgic-its.c
> index f162206adb48..ce3e3ed3f29f 100644
> --- a/arch/arm64/kvm/vgic/vgic-its.c
> +++ b/arch/arm64/kvm/vgic/vgic-its.c
> @@ -78,7 +78,6 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
>  {
>  	struct vgic_dist *dist = &kvm->arch.vgic;
>  	struct vgic_irq *irq = vgic_get_irq(kvm, intid), *oldirq;
> -	unsigned long flags;
>  	int ret;
>  
>  	/* In this case there is no put, since we keep the reference. */
> @@ -89,7 +88,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
>  	if (!irq)
>  		return ERR_PTR(-ENOMEM);
>  
> -	ret = xa_reserve_irq(&dist->lpi_xa, intid, GFP_KERNEL_ACCOUNT);
> +	ret = xa_reserve(&dist->lpi_xa, intid, GFP_KERNEL_ACCOUNT);
>  	if (ret) {
>  		kfree(irq);
>  		return ERR_PTR(ret);
> @@ -104,7 +103,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
>  	irq->target_vcpu = vcpu;
>  	irq->group = 1;
>  
> -	xa_lock_irqsave(&dist->lpi_xa, flags);
> +	xa_lock(&dist->lpi_xa);
>  
>  	/*
>  	 * There could be a race with another vgic_add_lpi(), so we need to
> @@ -126,7 +125,7 @@ static struct vgic_irq *vgic_add_lpi(struct kvm *kvm, u32 intid,
>  	}
>  
>  out_unlock:
> -	xa_unlock_irqrestore(&dist->lpi_xa, flags);
> +	xa_unlock(&dist->lpi_xa);
>  
>  	if (ret)
>  		return ERR_PTR(ret);
> diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> index 480391a0dcad..a21b482844ce 100644
> --- a/arch/arm64/kvm/vgic/vgic.c
> +++ b/arch/arm64/kvm/vgic/vgic.c
> @@ -28,7 +28,7 @@ struct vgic_global kvm_vgic_global_state __ro_after_init = {
>   *     kvm->arch.config_lock (mutex)
>   *       its->cmd_lock (mutex)
>   *         its->its_lock (mutex)
> - *           vgic_dist->lpi_xa.xa_lock		must be taken with IRQs disabled
> + *           vgic_dist->lpi_xa.xa_lock

Doesn't this contradict the commit message statement that we always
take this lock with interrupts disabled?

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.

Re: [PATCH 4/5] KVM: arm64: vgic-v3: Don't require IRQs be disabled for LPI xarray lock

[Oliver Upton]

On Fri, Sep 05, 2025 at 09:13:45AM +0100, Marc Zyngier wrote:
> > diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> > index 480391a0dcad..a21b482844ce 100644
> > --- a/arch/arm64/kvm/vgic/vgic.c
> > +++ b/arch/arm64/kvm/vgic/vgic.c
> > @@ -28,7 +28,7 @@ struct vgic_global kvm_vgic_global_state __ro_after_init = {
> >   *     kvm->arch.config_lock (mutex)
> >   *       its->cmd_lock (mutex)
> >   *         its->its_lock (mutex)
> > - *           vgic_dist->lpi_xa.xa_lock		must be taken with IRQs disabled
> > + *           vgic_dist->lpi_xa.xa_lock
> 
> Doesn't this contradict the commit message statement that we always
> take this lock with interrupts disabled?

[ relaying offline conversation ]

The changelog sucks hard on this one, what I was trying to say is there
are no longer any writers that acquire the xa_lock with IRQs disabled.
What remains is readers that have interrupts disabled, although readers
are RCU-protected and don't take the lock.

Thanks,
Oliver

[PATCH 5/5] KVM: arm64: vgic-v3: Indicate vgic_put_irq() may take LPI xarray lock

[Oliver Upton]

The release path on LPIs is quite rare, meaning it can be difficult to
find lock ordering bugs on the LPI xarray's spinlock. Tell lockdep that
vgic_put_irq() might acquire the xa_lock to make unsafe patterns more
obvious.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
 arch/arm64/kvm/vgic/vgic.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
index a21b482844ce..e61b95167ed1 100644
--- a/arch/arm64/kvm/vgic/vgic.c
+++ b/arch/arm64/kvm/vgic/vgic.c
@@ -142,6 +142,9 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
 {
 	struct vgic_dist *dist = &kvm->arch.vgic;
 
+	if (irq->intid > VGIC_MIN_LPI)
+		might_lock(&dist->lpi_xa.xa_lock);
+
 	if (!__vgic_put_irq(kvm, irq))
 		return;
 
-- 
2.39.5

Re: [PATCH 5/5] KVM: arm64: vgic-v3: Indicate vgic_put_irq() may take LPI xarray lock

[Ben Horgan]

Hi Oliver,

On 9/4/25 07:23, Oliver Upton wrote:
> The release path on LPIs is quite rare, meaning it can be difficult to
> find lock ordering bugs on the LPI xarray's spinlock. Tell lockdep that
> vgic_put_irq() might acquire the xa_lock to make unsafe patterns more
> obvious.
> 
> Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> ---
>  arch/arm64/kvm/vgic/vgic.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> index a21b482844ce..e61b95167ed1 100644
> --- a/arch/arm64/kvm/vgic/vgic.c
> +++ b/arch/arm64/kvm/vgic/vgic.c
> @@ -142,6 +142,9 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
>  {
>  	struct vgic_dist *dist = &kvm->arch.vgic;
>  
> +	if (irq->intid > VGIC_MIN_LPI)
> +		might_lock(&dist->lpi_xa.xa_lock);
> +

nit: Use >= rather than > to include all LPI.

>  	if (!__vgic_put_irq(kvm, irq))
>  		return;
>  

Thanks,

Ben

Re: [PATCH 5/5] KVM: arm64: vgic-v3: Indicate vgic_put_irq() may take LPI xarray lock

[Oliver Upton]

On Thu, Sep 04, 2025 at 11:25:08AM +0100, Ben Horgan wrote:
> Hi Oliver,
> 
> On 9/4/25 07:23, Oliver Upton wrote:
> > The release path on LPIs is quite rare, meaning it can be difficult to
> > find lock ordering bugs on the LPI xarray's spinlock. Tell lockdep that
> > vgic_put_irq() might acquire the xa_lock to make unsafe patterns more
> > obvious.
> > 
> > Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
> > ---
> >  arch/arm64/kvm/vgic/vgic.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/arch/arm64/kvm/vgic/vgic.c b/arch/arm64/kvm/vgic/vgic.c
> > index a21b482844ce..e61b95167ed1 100644
> > --- a/arch/arm64/kvm/vgic/vgic.c
> > +++ b/arch/arm64/kvm/vgic/vgic.c
> > @@ -142,6 +142,9 @@ void vgic_put_irq(struct kvm *kvm, struct vgic_irq *irq)
> >  {
> >  	struct vgic_dist *dist = &kvm->arch.vgic;
> >  
> > +	if (irq->intid > VGIC_MIN_LPI)
> > +		might_lock(&dist->lpi_xa.xa_lock);
> > +
> 
> nit: Use >= rather than > to include all LPI.

Of course, silly typo.

Thanks!
Oliver

Re: [PATCH 0/5] KVM: arm64: vgic-v3: Fix yet another lock ordering turd

[Marc Zyngier]

On Thu, 04 Sep 2025 07:23:43 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> syzkaller found yet another locking bug in the VGIC [*], this time due
> to nesting a 'plain' spinlock (xa_lock) inside of a raw spinlock
> (ap_list_lock). Given the way we do refcounts on LPIs it is possible
> for this exact sort of issue to crop up where the last reference may
> be dropped in unexpected places.
> 
> Small series to fix the issue by deferring xarray modifications outside
> of the ap_list_lock critical section along with some slight lockdep
> hinting to make these rare bugs a bit more obvious.

I quite like that we get a release operation, rather than the slightly
counter-intuitive construct we have today.

With the couple of issues that Ben and I raised fixed:

Reviewed-by: Marc Zyngier <maz@kernel.org>

	M.

-- 
Jazz isn't dead. It just smells funny.