[PATCH v4 1/2] bdi, blk-cgroup: Fix potential UAF of blkcg

[Waiman Long]

Commit 59b57717fff8 ("blkcg: delay blkg destruction until after
writeback has finished") delayed call to blkcg_destroy_blkgs() to
cgwb_release_workfn(). However, it is done after a css_put() of blkcg
which may be the final put that causes the blkcg to be freed as RCU
read lock isn't held.

Another place where blkcg_destroy_blkgs() can be called indirectly via
blkcg_unpin_online() is from the offline_css() function called from
css_killed_work_fn(). Over there, the potentially final css_put() call
is issued after offline_css().

By adding a css_tryget() into blkcg_destroy_blkgs() and warning its
failure, the following stack trace was produced in a test system on
bootup.

[   34.254240] RIP: 0010:blkcg_destroy_blkgs+0x16a/0x1a0
      :
[   34.339943] Call Trace:
[   34.344510]  blkcg_unpin_online+0x38/0x60
[   34.348523]  cgwb_release_workfn+0x6a/0x200
[   34.352708]  process_one_work+0x1e5/0x3b0
[   34.360758]  worker_thread+0x50/0x3a0
[   34.368447]  kthread+0xd9/0x100
[   34.376386]  ret_from_fork+0x22/0x30

This confirms that a potential UAF situation can really happen in
cgwb_release_workfn().

Fix that by delaying the css_put() until after the blkcg_unpin_online()
call. Also use percpu_ref_is_zero() in blkcg_destroy_blkgs() and issue
a warning if reference count is zero.

The reproducing system can no longer produce a warning with this patch.
All the runnable block/0* tests including block/027 were run successfully
without failure.

Fixes: 59b57717fff8 ("blkcg: delay blkg destruction until after writeback has finished")
Suggested-by: Michal Koutný <mkoutny@xxxxxxxx>
Reported-by: Yi Zhang <yi.zhang@xxxxxxxxxx>
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
Acked-by:  Tejun Heo <tj@xxxxxxxxxx>
---
 block/blk-cgroup.c | 7 +++++++
 mm/backing-dev.c   | 8 ++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 1bb939d3b793..ca28306aa1b1 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1084,6 +1084,13 @@ struct list_head *blkcg_get_cgwb_list(struct cgroup_subsys_state *css)
  */
 static void blkcg_destroy_blkgs(struct blkcg *blkcg)
 {
+	/*
+	 * blkcg_destroy_blkgs() shouldn't be called with all the blkcg
+	 * references gone.
+	 */
+	if (WARN_ON_ONCE(percpu_ref_is_zero(&blkcg->css.refcnt)))
+		return;
+
 	might_sleep();
 
 	spin_lock_irq(&blkcg->lock);
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index c30419a5e119..36f75b072325 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -390,11 +390,15 @@ static void cgwb_release_workfn(struct work_struct *work)
 	wb_shutdown(wb);
 
 	css_put(wb->memcg_css);
-	css_put(wb->blkcg_css);
 	mutex_unlock(&wb->bdi->cgwb_release_mutex);
 
-	/* triggers blkg destruction if no online users left */
+	/*
+	 * Triggers blkg destruction if no online users left
+	 * The final blkcg css_put() has to be done after blkcg_unpin_online()
+	 * to avoid use-after-free.
+	 */
 	blkcg_unpin_online(wb->blkcg_css);
+	css_put(wb->blkcg_css);
 
 	fprop_local_destroy_percpu(&wb->memcg_completions);
 
-- 
2.31.1