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block-backend: prevent dangling BDS pointers across aio_poll()

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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().

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2021778
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2036178
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20220111153613.25453-2-stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2022-01-11 15:36:12 +00:00 committed by Kevin Wolf
parent bb01ea7311
commit 1e3552dbd2
1 changed files with 17 additions and 2 deletions
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19
block/block-backend.c
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@ -822,16 +822,22 @@ BlockBackend *blk_by_public(BlockBackendPublic *public)
void blk_remove_bs(BlockBackend *blk)
{
ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
BlockDriverState *bs;
BdrvChild *root;
notifier_list_notify(&blk->remove_bs_notifiers, blk);
if (tgm->throttle_state) {
bs = blk_bs(blk);
BlockDriverState *bs = blk_bs(blk);
/*
* Take a ref in case blk_bs() changes across bdrv_drained_begin(), for
* example, if a temporary filter node is removed by a blockjob.
*/
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 +1711,7 @@ void blk_drain(BlockBackend *blk)
BlockDriverState *bs = blk_bs(blk);
if (bs) {
bdrv_ref(bs);
bdrv_drained_begin(bs);
}
@ -1714,6 +1721,7 @@ void blk_drain(BlockBackend *blk)
if (bs) {
bdrv_drained_end(bs);
bdrv_unref(bs);
}
}
@ -2044,10 +2052,13 @@ 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 +2068,8 @@ 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 +2339,13 @@ 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);
}
}