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

[Marc Zyngier <marc.zyngier@arm.com>]

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.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...
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