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Date: Thu, 7 Jul 2016 18:57:24 -0400 (EDT)
From: Mikulas Patocka <mpatocka@...hat.com>
To: Peter Hurley <peter@...leysoftware.com>
cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jslaby@...e.com>, linux-kernel@...r.kernel.org
Subject: Re: tty crash in Linux 4.6
Hi
This patch works, I've had no tty crashes since applying it.
I've seen that you haven't sent this patch yet to Linux-4.7-rc and
Linux-4.6-stable. Will you? Or did you create a different patch?
Mikulas
On Tue, 17 May 2016, Peter Hurley wrote:
> On 05/17/2016 08:57 AM, Peter Hurley wrote:
> > On 05/16/2016 04:36 PM, Peter Hurley wrote:
> >> > Hi Mikulas,
> >> >
> >> > On 05/16/2016 01:12 PM, Mikulas Patocka wrote:
> >>> >> Hi
> >>> >>
> >>> >> In the kernel 4.6 I get crashes in the tty layer. I can reproduce the
> >>> >> crash by logging into the machine with ssh and typing before the prompt
> >>> >> appears.
> >> >
> >> > Thanks for the report.
> >> > I tried to reproduce this a number of times on different machines
> >> > with no luck.
> >
> > I was able to reproduce this crash with a test jig.
> > The patch below fixed it, but I'm testing a better patch now, which
> > I'll get to you asap.
>
> --- >% ---
> 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 cdd063f..bda0c85 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;
> }
>
> --
> 2.8.2
>
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