FAILED: patch "[PATCH] writeback: fix use-after-free in finish_writeback_work()" failed to apply to 5.3-stable tree

FAILED: patch "[PATCH] writeback: fix use-after-free in finish_writeback_work()" failed to apply to 5.3-stable tree

[gregkh]

The patch below does not apply to the 5.3-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 <stable@vger.kernel.org>.

thanks,

greg k-h

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

From 8e00c4e9dd852f7a9bf12234fad65a2f2f93788f Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Sun, 6 Oct 2019 17:58:09 -0700
Subject: [PATCH] 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@devbig004.ftw2.facebook.com
Fixes: 5b9cce4c7eb069 ("writeback: Generalize and expose wb_completion")
Signed-off-by: Tejun Heo <tj@kernel.org>
Debugged-by: Chris Mason <clm@fb.com>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: <stable@vger.kernel.org>	[5.2+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 8aaa7eec7b74..e88421d9a48d 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -164,8 +164,13 @@ static void finish_writeback_work(struct bdi_writeback *wb,
 
 	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,

Re: FAILED: patch "[PATCH] writeback: fix use-after-free in finish_writeback_work()" failed to apply to 5.3-stable tree

[Sasha Levin]

On Sat, Oct 12, 2019 at 10:17:58AM +0200, gregkh@linuxfoundation.org wrote:
>
>The patch below does not apply to the 5.3-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 <stable@vger.kernel.org>.
>
>thanks,
>
>greg k-h
>
>------------------ original commit in Linus's tree ------------------
>
>From 8e00c4e9dd852f7a9bf12234fad65a2f2f93788f Mon Sep 17 00:00:00 2001
>From: Tejun Heo <tj@kernel.org>
>Date: Sun, 6 Oct 2019 17:58:09 -0700
>Subject: [PATCH] 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@devbig004.ftw2.facebook.com
>Fixes: 5b9cce4c7eb069 ("writeback: Generalize and expose wb_completion")

Hm... 5b9cce4c7eb069 went upstream during the 5.4 merge window, but:

>Signed-off-by: Tejun Heo <tj@kernel.org>
>Debugged-by: Chris Mason <clm@fb.com>
>Reviewed-by: Jens Axboe <axboe@kernel.dk>
>Cc: Jan Kara <jack@suse.cz>
>Cc: <stable@vger.kernel.org>	[5.2+]

This tag says that 8e00c4e9dd85 should be backported to 5.3.

-- 
Thanks,
Sasha

Re: FAILED: patch "[PATCH] writeback: fix use-after-free in finish_writeback_work()" failed to apply to 5.3-stable tree

[Linus Torvalds]

On Sat, Oct 12, 2019 at 1:18 AM <gregkh@linuxfoundation.org> wrote:
>
> The patch below does not apply to the 5.3-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 <stable@vger.kernel.org>.

Hmm. It looks to me like 5.3 doesn't have the use-after-free problem,
and that it was introduced by 5b9cce4c7eb0696558 ("writeback:
Generalize and expose wb_completion")

So I think the "Fixes" tag is right, and the "Cc: stable" is wrong.

            Linus

Re: FAILED: patch "[PATCH] writeback: fix use-after-free in finish_writeback_work()" failed to apply to 5.3-stable tree

[Tejun Heo]

On Sat, Oct 12, 2019 at 02:54:26PM -0700, Linus Torvalds wrote:
> On Sat, Oct 12, 2019 at 1:18 AM <gregkh@linuxfoundation.org> wrote:
> >
> > The patch below does not apply to the 5.3-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 <stable@vger.kernel.org>.
> 
> Hmm. It looks to me like 5.3 doesn't have the use-after-free problem,
> and that it was introduced by 5b9cce4c7eb0696558 ("writeback:
> Generalize and expose wb_completion")
> 
> So I think the "Fixes" tag is right, and the "Cc: stable" is wrong.

Yeap, sorry about that.  I think I forgot --contains when running
git-describe and mindlessly copied the output.

Thanks.

-- 
tejun