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

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

On Thu, Jun 06, 2019 at 01:06:33PM +0100, Marc Zyngier wrote:
> On 06/06/2019 11:58, Dave Martin wrote:
> > 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...
> 
> Yeah, this doesn't make much sense. Both timer and PMU are using PPIs,
> which are not refcounted, so this vgic_put_irq() is effectively a NOP.
> It doesn't invalidate the patch itself, it is just that I seriously
> doubt it fixes anything.
> 
> LPIs do not use the owner field so far, so we must have another get/put
> mismatch somewhere.

No argument from me.

As I say, this change _appeared_ to make this leak go away, but I
couldn't understand why, and didn't kick it very thoroughly.  So it
may well be a red herring.

Cheers
---Dave
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