[PATCH v2] writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs

[Baokun Li]

KASAN report null-ptr-deref:
==================================================================
BUG: KASAN: null-ptr-deref in bdi_split_work_to_wbs+0x5c5/0x7b0
Write of size 8 at addr 0000000000000000 by task sync/943
CPU: 5 PID: 943 Comm: sync Tainted: 6.3.0-rc5-next-20230406-dirty #461
Call Trace:
 <TASK>
 dump_stack_lvl+0x7f/0xc0
 print_report+0x2ba/0x340
 kasan_report+0xc4/0x120
 kasan_check_range+0x1b7/0x2e0
 __kasan_check_write+0x24/0x40
 bdi_split_work_to_wbs+0x5c5/0x7b0
 sync_inodes_sb+0x195/0x630
 sync_inodes_one_sb+0x3a/0x50
 iterate_supers+0x106/0x1b0
 ksys_sync+0x98/0x160
[...]
==================================================================

The race that causes the above issue is as follows:

           cpu1                     cpu2
-------------------------|-------------------------
inode_switch_wbs
 INIT_WORK(&isw->work, inode_switch_wbs_work_fn)
 queue_rcu_work(isw_wq, &isw->work)
 // queue_work async
  inode_switch_wbs_work_fn
   wb_put_many(old_wb, nr_switched)
    percpu_ref_put_many
     ref->data->release(ref)
     cgwb_release
      queue_work(cgwb_release_wq, &wb->release_work)
      // queue_work async
       &wb->release_work
       cgwb_release_workfn
                            ksys_sync
                             iterate_supers
                              sync_inodes_one_sb
                               sync_inodes_sb
                                bdi_split_work_to_wbs
                                 kmalloc(sizeof(*work), GFP_ATOMIC)
                                 // alloc memory failed
        percpu_ref_exit
         ref->data = NULL
         kfree(data)
                                 wb_get(wb)
                                  percpu_ref_get(&wb->refcnt)
                                   percpu_ref_get_many(ref, 1)
                                    atomic_long_add(nr, &ref->data->count)
                                     atomic64_add(i, v)
                                     // trigger null-ptr-deref

bdi_split_work_to_wbs() traverses &bdi->wb_list to split work into all wbs.
If the allocation of new work fails, the on-stack fallback will be used and
the reference count of the current wb is increased afterwards. If cgroup
writeback membership switches occur before getting the reference count and
the current wb is released as old_wd, then calling wb_get() or wb_put()
will trigger the null pointer dereference above.

This issue was introduced in v4.3-rc7 (see fix tag1). Both sync_inodes_sb()
and __writeback_inodes_sb_nr() calls to bdi_split_work_to_wbs() can trigger
this issue. For scenarios called via sync_inodes_sb(), originally commit
7fc5854f8c6e ("writeback: synchronize sync(2) against cgroup writeback
membership switches") reduced the possibility of the issue by adding
wb_switch_rwsem, but in v5.14-rc1 (see fix tag2) removed the
"inode_io_list_del_locked(inode, old_wb)" from inode_switch_wbs_work_fn()
so that wb->state contains WB_has_dirty_io, thus old_wb is not skipped
when traversing wbs in bdi_split_work_to_wbs(), and the issue becomes
easily reproducible again.

To solve this problem, percpu_ref_exit() is called under RCU protection
to avoid race between cgwb_release_workfn() and bdi_split_work_to_wbs().
Moreover, replace wb_get() with wb_tryget() in bdi_split_work_to_wbs(),
and skip the current wb if wb_tryget() fails because the wb has already
been shutdown.

Fixes: b817525a4a80 ("writeback: bdi_writeback iteration must not skip dying ones")
Fixes: f3b6a6df38aa ("writeback, cgroup: keep list of inodes attached to bdi_writeback")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
---
V1->V2:
	Use RCU instead of wb_switch_rwsem to avoid race.

 fs/fs-writeback.c | 17 ++++++++++-------
 mm/backing-dev.c  | 12 ++++++++++--
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 195dc23e0d83..1db3e3c24b43 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -978,6 +978,16 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 			continue;
 		}
 
+		/*
+		 * If wb_tryget fails, the wb has been shutdown, skip it.
+		 *
+		 * Pin @wb so that it stays on @bdi->wb_list.  This allows
+		 * continuing iteration from @wb after dropping and
+		 * regrabbing rcu read lock.
+		 */
+		if (!wb_tryget(wb))
+			continue;
+
 		/* alloc failed, execute synchronously using on-stack fallback */
 		work = &fallback_work;
 		*work = *base_work;
@@ -986,13 +996,6 @@ static void bdi_split_work_to_wbs(struct backing_dev_info *bdi,
 		work->done = &fallback_work_done;
 
 		wb_queue_work(wb, work);
-
-		/*
-		 * Pin @wb so that it stays on @bdi->wb_list.  This allows
-		 * continuing iteration from @wb after dropping and
-		 * regrabbing rcu read lock.
-		 */
-		wb_get(wb);
 		last_wb = wb;
 
 		rcu_read_unlock();
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index ad011308cebe..43b48750b491 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -507,6 +507,15 @@ static LIST_HEAD(offline_cgwbs);
 static void cleanup_offline_cgwbs_workfn(struct work_struct *work);
 static DECLARE_WORK(cleanup_offline_cgwbs_work, cleanup_offline_cgwbs_workfn);
 
+static void cgwb_free_rcu(struct rcu_head *rcu_head)
+{
+	struct bdi_writeback *wb = container_of(rcu_head,
+			struct bdi_writeback, rcu);
+
+	percpu_ref_exit(&wb->refcnt);
+	kfree(wb);
+}
+
 static void cgwb_release_workfn(struct work_struct *work)
 {
 	struct bdi_writeback *wb = container_of(work, struct bdi_writeback,
@@ -529,11 +538,10 @@ static void cgwb_release_workfn(struct work_struct *work)
 	list_del(&wb->offline_node);
 	spin_unlock_irq(&cgwb_lock);
 
-	percpu_ref_exit(&wb->refcnt);
 	wb_exit(wb);
 	bdi_put(bdi);
 	WARN_ON_ONCE(!list_empty(&wb->b_attached));
-	kfree_rcu(wb, rcu);
+	call_rcu(&wb->rcu, cgwb_free_rcu);
 }
 
 static void cgwb_release(struct percpu_ref *refcnt)
-- 
2.31.1