Re: [Qemu-devel] [PATCH] qcow2: fix double-free of Qcow2DiscardRegion in qcow2_process_discards

[Zhang Haoyu <ahzhanghaoyu@gmail.com>]

On 2014-10-12 15:34, Kevin Wolf wrote:
> Am 11.10.2014 um 09:14 hat Zhang Haoyu geschrieben:
>> In qcow2_update_snapshot_refcount -> qcow2_process_discards() -> bdrv_discard()
>> may free the Qcow2DiscardRegion which is referenced by "next" pointer in
>> qcow2_process_discards() now, in next iteration, d = next, so g_free(d)
>> will double-free this Qcow2DiscardRegion.
>>
>> qcow2_snapshot_delete
>> |- qcow2_update_snapshot_refcount
>> |-- qcow2_process_discards
>> |--- bdrv_discard
>> |---- aio_poll
>> |----- aio_dispatch
>> |------ bdrv_co_io_em_complete
>> |------- qemu_coroutine_enter(co->coroutine, NULL); <=== coroutine entry is bdrv_co_do_rw
>> |--- g_free(d) <== free first Qcow2DiscardRegion is okay
>> |--- d = next;  <== this set is done in QTAILQ_FOREACH_SAFE() macro.
>> |--- g_free(d);  <== double-free will happen if during previous iteration, bdrv_discard had free this object.
> Do you have a reproducer for this or did code review lead you to this?
This problem can be reproduced with loop of savevm -> delvm -> savem -> 
delvm ..., about 4 hours.
When I delete the vm snapshot, qemu crashed with a core file,
I debug the core file and find the double-free and the stack.

So I add a breakpoint at g_free(d);, and find that indeed a double-free 
happened,  twice free with the same address.
And only the first discard region have not happened with double-free.
>
> At the moment I can't see how bdrv_discard(bs->file) could ever free a
> Qcow2DiscardRegion of bs, as it's working on a completely different
> BlockDriverState (which usually won't even be a qcow2 one).
I think the "aio_context" in bdrv_discard -> aio_poll(aio_context, true) 
is the qemu_aio_context,
no matter the bs or bs->file passed to bdrv_discard, so 
aio_poll(aio_context) will poll all of the aio.
>
>> bdrv_co_do_rw
>> |- bdrv_co_do_writev
>> |-- bdrv_co_do_pwritev
>> |--- bdrv_aligned_pwritev
>> |---- qcow2_co_writev
>> |----- qcow2_alloc_cluster_link_l2
>> |------ qcow2_free_any_clusters
>> |------- qcow2_free_clusters
>> |-------- update_refcount
>> |--------- qcow2_process_discards
>> |---------- g_free(d)  <== In next iteration, this Qcow2DiscardRegion will be double-free.
> This shouldn't happen in a nested call either, as s->lock can't be taken
> recursively.
Could you detail how s->lock prevent that, above stack is from the gdb, 
when I add a breakpoint in g_free(d).

Thanks,
Zhang Haoyu
>
> Kevin
>
>