[PATCH 2/2] KVM: arm/arm64: vgic: Fix irq refcount leak in kvm_vgic_set_owner()

[Dave Martin <Dave.Martin@arm.com>]

kvm_vgic_set_owner() leaks a reference on the vgic_irq descriptor,
which does not seem to match up with any vgic_put_irq() that I can
find.

Since the irq pointer is not passed out and the caller must anyway
subsequently use vgic_get_irq() when is wants a pointer, it is not
clear why we should have a dangling refcount here.

The refcount is still needed inside kvm_vgic_set_owner() to prevent
the vgic_irq struct from disappearing while while it is
manipulated.

So, keep it vgic_get_irq() here, but add the matching
vgic_put_irq() before returning.

unreferenced object 0xffff800b6365ab80 (size 128):
  comm "qemu-system-aar", pid 14414, jiffies 4300822606 (age 84.436s)
  hex dump (first 32 bytes):
    00 00 00 00 00 00 00 00 b0 e1 e0 38 00 00 ff ff  ...........8....
    b0 e1 e0 38 00 00 ff ff 78 e6 ad dd 0a 80 ff ff  ...8....x.......
  backtrace:
    [<00000000a08b80e2>] kmem_cache_alloc+0x178/0x208
    [<00000000114591cb>] vgic_add_lpi.part.5+0x34/0x190
    [<00000000ec1425ae>] vgic_its_cmd_handle_mapi+0x320/0x348
    [<00000000935c5c32>] vgic_its_process_commands.part.14+0x350/0x8b8
    [<00000000dc256d2c>] vgic_mmio_write_its_cwriter+0x78/0x98
    [<000000008659acd2>] dispatch_mmio_write+0xd4/0x120

[...]

Cc: Christoffer Dall <christoffer.dall@arm.com>
Fixes: c6ccd30e0de3 ("KVM: arm/arm64: Introduce an allocator for in-kernel irq lines")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>

---

Based on the limited testing I've done so far, the patch _appears_ to
fix the bug.

However, I still don't understand which the bug is intermittent, or why
the arch_timer or pmu (the only apparent users of kvm_vgic_set_owner())
are claiming an LPI in the first place.

So there may be other bugs in the mix, or I may have misunderstood
something...

The bug (and fix) were observed with native qemu on ThunderX2, on a
merge of v5.1 with kvmarm/next commit 9eecfc22e0bf ("KVM: arm64: Fix
ptrauth ID register masking logic").

My qemu invocation was:

$ qemu-system-aarch64 -machine virt,accel=kvm,gic_version=3 -cpu host \
    -smp 4 -nographic \
    -drive id=vblock,file=block.qcow2,format=qcow2,if=none \
    -device virtio-blk-device,drive=vblock \
    -kernel Image -append 'root=/dev/vda1 ro'
---
 virt/kvm/arm/vgic/vgic.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/virt/kvm/arm/vgic/vgic.c b/virt/kvm/arm/vgic/vgic.c
index 191decc..930319c 100644
--- a/virt/kvm/arm/vgic/vgic.c
+++ b/virt/kvm/arm/vgic/vgic.c
@@ -599,6 +599,7 @@ int kvm_vgic_set_owner(struct kvm_vcpu *vcpu, unsigned int intid, void *owner)
 	else
 		irq->owner = owner;
 	raw_spin_unlock_irqrestore(&irq->irq_lock, flags);
+	vgic_put_irq(vcpu->kvm, irq);
 
 	return ret;
 }
-- 
2.1.4

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