[PATCH AUTOSEL 5.4 26/26] writeback, cgroup: increment isw_nr_in_flight before grabbing an inode

[Sasha Levin]

From: Roman Gushchin <guro@xxxxxx>

[ Upstream commit 8826ee4fe75051f8cbfa5d4a9aa70565938e724c ]

isw_nr_in_flight is used to determine whether the inode switch queue
should be flushed from the umount path.  Currently it's increased after
grabbing an inode and even scheduling the switch work.  It means the
umount path can walk past cleanup_offline_cgwb() with active inode
references, which can result in a "Busy inodes after unmount." message and
use-after-free issues (with inode->i_sb which gets freed).

Fix it by incrementing isw_nr_in_flight before doing anything with the
inode and decrementing in the case when switching wasn't scheduled.

The problem hasn't yet been seen in the real life and was discovered by
Jan Kara by looking into the code.

Link: https://lkml.kernel.org/r/20210608230225.2078447-4-guro@xxxxxx
Signed-off-by: Roman Gushchin <guro@xxxxxx>
Suggested-by: Jan Kara <jack@xxxxxxxx>
Reviewed-by: Jan Kara <jack@xxxxxxx>
Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Dave Chinner <dchinner@xxxxxxxxxx>
Cc: Dennis Zhou <dennis@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 fs/fs-writeback.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index fd6b50582c87..3a0d7b8af141 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -505,6 +505,8 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	if (!isw)
 		return;
 
+	atomic_inc(&isw_nr_in_flight);
+
 	/* find and pin the new wb */
 	rcu_read_lock();
 	memcg_css = css_from_id(new_wb_id, &memory_cgrp_subsys);
@@ -535,11 +537,10 @@ static void inode_switch_wbs(struct inode *inode, int new_wb_id)
 	 * Let's continue after I_WB_SWITCH is guaranteed to be visible.
 	 */
 	call_rcu(&isw->rcu_head, inode_switch_wbs_rcu_fn);
-
-	atomic_inc(&isw_nr_in_flight);
 	return;
 
 out_free:
+	atomic_dec(&isw_nr_in_flight);
 	if (isw->new_wb)
 		wb_put(isw->new_wb);
 	kfree(isw);
-- 
2.30.2