[PATCH 4.19 85/93] KVM: arm/arm64: vgic: Fix potential deadlock when ap_list is long

[Greg Kroah-Hartman]

[ Upstream commit d4a8061a7c5f7c27a2dc002ee4cb89b3e6637e44 ]

If the ap_list is longer than 256 entries, merge_final() in list_sort()
will call the comparison callback with the same element twice, causing
a deadlock in vgic_irq_cmp().

Fix it by returning early when irqa == irqb.

Cc: stable@xxxxxxxxxxxxxxx # 4.7+
Fixes: 8e4447457965 ("KVM: arm/arm64: vgic-new: Add IRQ sorting")
Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
Signed-off-by: Heyi Guo <guoheyi@xxxxxxxxxx>
[maz: massaged commit log and patch, added Fixes and Cc-stable]
Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 virt/kvm/arm/vgic/vgic.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
index 250cd72c95a52..4040a33cdc902 100644
--- a/virt/kvm/arm/vgic/vgic.c
+++ b/virt/kvm/arm/vgic/vgic.c
@@ -244,6 +244,13 @@ static int vgic_irq_cmp(void *priv, struct list_head *a, struct list_head *b)
 	bool penda, pendb;
 	int ret;
 
+	/*
+	 * list_sort may call this function with the same element when
+	 * the list is fairly long.
+	 */
+	if (unlikely(irqa == irqb))
+		return 0;
+
 	spin_lock(&irqa->irq_lock);
 	spin_lock_nested(&irqb->irq_lock, SINGLE_DEPTH_NESTING);
 
-- 
2.20.1