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

[Hanna Reitz]

On 14.12.21 15:35, Stefan Hajnoczi wrote:
> 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.

By the way, I have a BZ for this, though it’s about block-stream instead 
of backup (https://bugzilla.redhat.com/show_bug.cgi?id=2036178).  But 
I’m happy to report your patch seems* to fix that problem, too!  (Thanks 
for fixing my BZs! :))

*I’ve written a reproducer in iotest form 
(https://gitlab.com/hreitz/qemu/-/blob/stefans-fix-and-a-test/tests/qemu-iotests/tests/stream-error-on-reset), 
and so far I can only assume it indeed reproduces the report, but I 
found that iotest to indeed be fixed by this patch.  (Which made me very 
happy.)

Hanna

> 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);
> @@ -1705,6 +1707,7 @@ void blk_drain(BlockBackend *blk)
>       BlockDriverState *bs = blk_bs(blk);
>   
>       if (bs) {
> +        bdrv_ref(bs);
>           bdrv_drained_begin(bs);
>       }
>   
> @@ -1714,6 +1717,7 @@ void blk_drain(BlockBackend *blk)
>   
>       if (bs) {
>           bdrv_drained_end(bs);
> +        bdrv_unref(bs);
>       }
>   }
>   
> @@ -2044,10 +2048,13 @@ static int blk_do_set_aio_context(BlockBackend *blk, AioContext *new_context,
>       int ret;
>   
>       if (bs) {
> +        bdrv_ref(bs);
> +
>           if (update_root_node) {
>               ret = bdrv_child_try_set_aio_context(bs, new_context, blk->root,
>                                                    errp);
>               if (ret < 0) {
> +                bdrv_unref(bs);
>                   return ret;
>               }
>           }
> @@ -2057,6 +2064,8 @@ static int blk_do_set_aio_context(BlockBackend *blk, 
> AioContext *new_context,
>               throttle_group_attach_aio_context(tgm, new_context);
>               bdrv_drained_end(bs);
>           }
> +
> +        bdrv_unref(bs);
>       }
>   
>       blk->ctx = new_context;
> @@ -2326,11 +2335,13 @@ void blk_io_limits_disable(BlockBackend *blk)
>       ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
>       assert(tgm->throttle_state);
>       if (bs) {
> +        bdrv_ref(bs);
>           bdrv_drained_begin(bs);
>       }
>       throttle_group_unregister_tgm(tgm);
>       if (bs) {
>           bdrv_drained_end(bs);
> +        bdrv_unref(bs);
>       }
>   }
>