Re: [PATCH v2] block-backend: prevent dangling BDS pointers across aio_poll()

[Stefan Hajnoczi]

On Tue, Dec 14, 2021 at 03:59:49PM +0100, Kevin Wolf wrote:
> Am 14.12.2021 um 15:35 hat Stefan Hajnoczi geschrieben:
> > The BlockBackend root child can change when aio_poll() is invoked. This
> > happens when a temporary filter node is removed upon blockjob
> > completion, for example.
> > 
> > Functions in block/block-backend.c must be aware of this when using a
> > blk_bs() pointer across aio_poll() because the BlockDriverState refcnt
> > may reach 0, resulting in a stale pointer.
> > 
> > One example is scsi_device_purge_requests(), which calls blk_drain() to
> > wait for in-flight requests to cancel. If the backup blockjob is active,
> > then the BlockBackend root child is a temporary filter BDS owned by the
> > blockjob. The blockjob can complete during bdrv_drained_begin() and the
> > last reference to the BDS is released when the temporary filter node is
> > removed. This results in a use-after-free when blk_drain() calls
> > bdrv_drained_end(bs) on the dangling pointer.
> > 
> > Explicitly hold a reference to bs across block APIs that invoke
> > aio_poll().
> > 
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > v2:
> > - Audit block/block-backend.c and fix additional cases
> > ---
> >  block/block-backend.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> > 
> > diff --git a/block/block-backend.c b/block/block-backend.c
> > index 12ef80ea17..a40ad7fa92 100644
> > --- a/block/block-backend.c
> > +++ b/block/block-backend.c
> > @@ -828,10 +828,12 @@ void blk_remove_bs(BlockBackend *blk)
> >      notifier_list_notify(&blk->remove_bs_notifiers, blk);
> >      if (tgm->throttle_state) {
> >          bs = blk_bs(blk);
> > +        bdrv_ref(bs);
> >          bdrv_drained_begin(bs);
> >          throttle_group_detach_aio_context(tgm);
> >          throttle_group_attach_aio_context(tgm, qemu_get_aio_context());
> >          bdrv_drained_end(bs);
> > +        bdrv_unref(bs);
> >      }
> >  
> >      blk_update_root_state(blk);
> 
> This hunk is unnecessary, we still hold a reference that is only given
> up a few lines down with bdrv_root_unref_child(root).

That's not the only place where the reference can be dropped:
bdrv_drop_filter() removes the filter node from the graph.

Here is a case where it happens: block/backup.c:backup_clean() ->
bdrv_cbw_drop() -> bdrv_drop_filter() -> bdrv_replace_node_common() ->
bdrv_replace_child_commit(). After we reach this bdrv_unref() is called
a few times and all references are dropped because the node is no longer
in the graph.

This happens during blk_remove_bs() -> bdrv_drained_begin(), so the bs
pointer in the above hunk can be stale.

Stefan

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