Date:   Wed, 8 Feb 2017 15:34:07 -0500 (EST)
From:   Mikulas Patocka <mpatocka@...hat.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Hurley <peter@...leysoftware.com>,
        linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: status of "tty: Fix ldisc crash on reopened tty"

I can still trigger this crash on the kernel 4.10-rc6 and I confirm that 
the patch below fixes it.

Peter Hurley, the author of the patch, somehow stopped communicating about 
the patch and he didn't push it into the kernel. If you could get the 
patch into the kernel, do it.

The patch should be backported to stable kernels starting with 4.6.

Mikulas


On Sat, 4 Feb 2017, Sergey Senozhatsky wrote:

> 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>

Tested-by: Mikulas Patocka <mpatocka@...hat.com>
Cc: stable@...r.kernel.org	# 4.6+

> ---
>  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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