FAILED: patch "[PATCH] writeback, cgroup: fix null-ptr-deref write in" failed to apply to 4.14-stable tree

The patch below does not apply to the 4.14-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-4.14.y
git checkout FETCH_HEAD
git cherry-pick -x 1ba1199ec5747f475538c0d25a32804e5ba1dfde
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2023042229-enchilada-cheddar-abb9@gregkh' --subject-prefix 'PATCH 4.14.y' HEAD^..

Possible dependencies:

1ba1199ec574 ("writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs") c22d70a162d3 ("writeback, cgroup: release dying cgwbs by switching attached inodes") f3b6a6df38aa ("writeback, cgroup: keep list of inodes attached to bdi_writeback")
29264d92a0f1 ("writeback, cgroup: switch to rcu_work API in inode_switch_wbs()")
8826ee4fe750 ("writeback, cgroup: increment isw_nr_in_flight before grabbing an inode") 4ade5867b4b8 ("writeback, cgroup: do not switch inodes with I_WILL_FREE flag")
e30942859030 ("Merge tag 'writeback_for_v5.9-rc3' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 1ba1199ec5747f475538c0d25a32804e5ba1dfde Mon Sep 17 00:00:00 2001
From: Baokun Li <[email protected]>
Date: Mon, 10 Apr 2023 21:08:26 +0800
Subject: [PATCH] writeback, cgroup: fix null-ptr-deref write in
bdi_split_work_to_wbs

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.

index 195dc23e0d83..1db3e3c24b43 100644
@@ -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();

index a53b9360b72e..30d2d0386fdb 100644
@@ -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)