+ writeback-fix-use-after-free-in-finish_writeback_work.patch added to -mm tree

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The patch titled
     Subject: writeback: fix use-after-free in finish_writeback_work()
has been added to the -mm tree.  Its filename is
     writeback-fix-use-after-free-in-finish_writeback_work.patch

This patch should soon appear at
    http://ozlabs.org/~akpm/mmots/broken-out/writeback-fix-use-after-free-in-finish_writeback_work.patch
and later at
    http://ozlabs.org/~akpm/mmotm/broken-out/writeback-fix-use-after-free-in-finish_writeback_work.patch

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------------------------------------------------------
From: Tejun Heo <tj@xxxxxxxxxx>
Subject: writeback: fix use-after-free in finish_writeback_work()

finish_writeback_work() reads @done->waitq after decrementing @done->cnt. 
However, once @done->cnt reaches zero, @done may be freed (from stack) at
any moment and @done->waitq can contain something unrelated by the time
finish_writeback_work() tries to read it.  This led to the following
crash.

  "BUG: kernel NULL pointer dereference, address: 0000000000000002"
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP DEBUG_PAGEALLOC
  CPU: 40 PID: 555153 Comm: kworker/u98:50 Kdump: loaded Not tainted
  ...
  Workqueue: writeback wb_workfn (flush-btrfs-1)
  RIP: 0010:_raw_spin_lock_irqsave+0x10/0x30
  Code: 48 89 d8 5b c3 e8 50 db 6b ff eb f4 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 53 9c 5b fa 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 05 48 89 d8 5b c3 89 c6 e8 fe ca 6b ff eb f2 66 90
  RSP: 0018:ffffc90049b27d98 EFLAGS: 00010046
  RAX: 0000000000000000 RBX: 0000000000000246 RCX: 0000000000000000
  RDX: 0000000000000001 RSI: 0000000000000003 RDI: 0000000000000002
  RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
  R10: ffff889fff407600 R11: ffff88ba9395d740 R12: 000000000000e300
  R13: 0000000000000003 R14: 0000000000000000 R15: 0000000000000000
  FS:  0000000000000000(0000) GS:ffff88bfdfa00000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000002 CR3: 0000000002409005 CR4: 00000000001606e0
  Call Trace:
   __wake_up_common_lock+0x63/0xc0
   wb_workfn+0xd2/0x3e0
   process_one_work+0x1f5/0x3f0
   worker_thread+0x2d/0x3d0
   kthread+0x111/0x130
   ret_from_fork+0x1f/0x30

Fix it by reading and caching @done->waitq before decrementing
@done->cnt.

Link: http://lkml.kernel.org/r/20190924010631.GH2233839@xxxxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 5b9cce4c7eb069 ("writeback: Generalize and expose wb_completion")
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Debugged-by: Chris Mason <clm@xxxxxx>
Reviewed-by: Jens Axboe <axboe@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>	[5.2+]
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 fs/fs-writeback.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

--- a/fs/fs-writeback.c~writeback-fix-use-after-free-in-finish_writeback_work
+++ a/fs/fs-writeback.c
@@ -164,8 +164,13 @@ static void finish_writeback_work(struct
 
 	if (work->auto_free)
 		kfree(work);
-	if (done && atomic_dec_and_test(&done->cnt))
-		wake_up_all(done->waitq);
+	if (done) {
+		wait_queue_head_t *waitq = done->waitq;
+
+		/* @done can't be accessed after the following dec */
+		if (atomic_dec_and_test(&done->cnt))
+			wake_up_all(waitq);
+	}
 }
 
 static void wb_queue_work(struct bdi_writeback *wb,
_

Patches currently in -mm which might be from tj@xxxxxxxxxx are

writeback-fix-use-after-free-in-finish_writeback_work.patch




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