Date:   Sat, 4 Feb 2017 19:23:25 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Peter Hurley <peter@...leysoftware.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: status of "tty: Fix ldisc crash on reopened tty"

On (02/04/17 09:09), Greg Kroah-Hartman wrote:
> On Sat, Feb 04, 2017 at 01:57:19PM +0900, Sergey Senozhatsky wrote:
> > Cc Greg
> > 
> > 
> > On (01/24/17 14:19), Sergey Senozhatsky wrote:
> > > Hello Peter, Mikulas
> > > 
> > > just came across this patch https://lkml.org/lkml/2016/5/17/440
> > > 
> > > Peter, are you still planning to merge it? or is there something
> > > that made you change your mind?
> > 
> > ping
> 
> worst email ever...

could be. sorry about that.


> Please include the patch if you want it applied, especially for
> something so old.

well, at this point I'm trying to find out why the patch has not been
submitted. may be Peter changed his mind or found something wrong with
the patch... I don't know. I though that questions I asked in my original
email (from 01/24/17) would point that out.


nevertheless, should have written my email in more 'clear' way.
here it comes:

I hit two Oops-es several weeks ago (4.1 LTE kernel) with backtraces
that look similar to the one mentioned in the patch below. I can't
reproduce the issue and can't verify the patch, but it looks reasonable.
since I couldn't test it, I didn't include Peter's patch. I Cc-d you
because you are undoubtedly experienced in TTY layer. Greg, is there any
chance you can take a look at the patch? for you convenience the patch
"included". once again, I personally can't verify it. OTOH, Mikulas
Patocka reported that the patch fixed the issue on his side.

===8<===8<===

>From aab3dd8e4877a5ca9327450dcba5c075f9e8f4c7 Mon Sep 17 00:00:00 2001
From: Peter Hurley <peter@...leysoftware.com>
Date: Sat, 4 Feb 2017 19:12:21 +0900
Subject: [PATCH] tty: Fix ldisc crash on reopened tty

If the tty has been hungup, the ldisc instance may have been destroyed.
Continued input to the tty will be ignored as long as the ldisc instance
is not visible to the flush_to_ldisc kworker. However, when the tty
is reopened and a new ldisc instance is created, the flush_to_ldisc
kworker can obtain an ldisc reference before the new ldisc is
completely initialized. This will likely crash:

 BUG: unable to handle kernel paging request at 0000000000002260
 IP: [<ffffffff8152dc5d>] n_tty_receive_buf_common+0x6d/0xb80
 PGD 2ab581067 PUD 290c11067 PMD 0
 Oops: 0000 [#1] PREEMPT SMP
 Modules linked in: nls_iso8859_1 ip6table_filter [.....]
 CPU: 2 PID: 103 Comm: kworker/u16:1 Not tainted 4.6.0-rc7+wip-xeon+debug #rc7+wip
 Hardware name: Dell Inc. Precision WorkStation T5400  /0RW203, BIOS A11 04/30/2012
 Workqueue: events_unbound flush_to_ldisc
 task: ffff8802ad16d100 ti: ffff8802ad31c000 task.ti: ffff8802ad31c000
 RIP: 0010:[<ffffffff8152dc5d>]  [<ffffffff8152dc5d>] n_tty_receive_buf_common+0x6d/0xb80
 RSP: 0018:ffff8802ad31fc70  EFLAGS: 00010296
 RAX: 0000000000000000 RBX: ffff8802aaddd800 RCX: 0000000000000001
 RDX: 00000000ffffffff RSI: ffffffff810db48f RDI: 0000000000000246
 RBP: ffff8802ad31fd08 R08: 0000000000000000 R09: 0000000000000001
 R10: ffff8802aadddb28 R11: 0000000000000001 R12: ffff8800ba6da808
 R13: ffff8802ad18be80 R14: ffff8800ba6da858 R15: ffff8800ba6da800
 FS:  0000000000000000(0000) GS:ffff8802b0a00000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000002260 CR3: 000000028ee5d000 CR4: 00000000000006e0
 Stack:
  ffffffff81531219 ffff8802aadddab8 ffff8802aadddde0 ffff8802aadddd78
  ffffffff00000001 ffff8800ba6da858 ffff8800ba6da860 ffff8802ad31fd30
  ffffffff81885f78 ffffffff81531219 0000000000000000 0000000200000000
 Call Trace:
  [<ffffffff81531219>] ? flush_to_ldisc+0x49/0xd0
  [<ffffffff81885f78>] ? mutex_lock_nested+0x2c8/0x430
  [<ffffffff81531219>] ? flush_to_ldisc+0x49/0xd0
  [<ffffffff8152e784>] n_tty_receive_buf2+0x14/0x20
  [<ffffffff81530cb2>] tty_ldisc_receive_buf+0x22/0x50
  [<ffffffff8153128e>] flush_to_ldisc+0xbe/0xd0
  [<ffffffff810a0ebd>] process_one_work+0x1ed/0x6e0
  [<ffffffff810a0e3f>] ? process_one_work+0x16f/0x6e0
  [<ffffffff810a13fe>] worker_thread+0x4e/0x490
  [<ffffffff810a13b0>] ? process_one_work+0x6e0/0x6e0
  [<ffffffff810a7ef2>] kthread+0xf2/0x110
  [<ffffffff810ae68c>] ? preempt_count_sub+0x4c/0x80
  [<ffffffff8188ab52>] ret_from_fork+0x22/0x50
  [<ffffffff810a7e00>] ? kthread_create_on_node+0x220/0x220
 Code: ff ff e8 27 a0 35 00 48 8d 83 78 05 00 00 c7 45 c0 00 00 00 00 48 89 45 80 48
       8d 83 e0 05 00 00 48 89 85 78 ff ff ff 48 8b 45 b8 <48> 8b b8 60 22 00 00 48
       8b 30 89 f8 8b 8b 88 04 00 00 29 f0 8d
 RIP  [<ffffffff8152dc5d>] n_tty_receive_buf_common+0x6d/0xb80
  RSP <ffff8802ad31fc70>
 CR2: 0000000000002260

Ensure the kworker cannot obtain the ldisc reference until the new ldisc
is completely initialized.

Fixes: 892d1fa7eaae ("tty: Destroy ldisc instance on hangup")
Reported-by: Mikulas Patocka <mpatocka@...hat.com>
Signed-off-by: Peter Hurley <peter@...leysoftware.com>
---
 drivers/tty/tty_ldisc.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c
index 68947f6de5ad..4ee7742dced3 100644
--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -669,16 +669,17 @@ int tty_ldisc_reinit(struct tty_struct *tty, int disc)
 		tty_ldisc_put(tty->ldisc);
 	}
 
-	/* switch the line discipline */
-	tty->ldisc = ld;
 	tty_set_termios_ldisc(tty, disc);
-	retval = tty_ldisc_open(tty, tty->ldisc);
+	retval = tty_ldisc_open(tty, ld);
 	if (retval) {
 		if (!WARN_ON(disc == N_TTY)) {
-			tty_ldisc_put(tty->ldisc);
-			tty->ldisc = NULL;
+			tty_ldisc_put(ld);
+			ld = NULL;
 		}
 	}
+
+	/* switch the line discipline */
+	smp_store_release(&tty->ldisc, ld);
 	return retval;
 }
 
-- 
2.11.1

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