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

[Zhang Haoyu]

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