Date:   Sat, 11 Mar 2017 11:31:43 +1100
From:   Michael Neuling <mikey@...ling.org>
To:     Mikulas Patocka <mpatocka@...hat.com>,
        Peter Hurley <peter@...leysoftware.com>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Michael Neuling <mikey@...ling.org>
Subject: Re: tty crash in Linux 4.6

> 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?

We are hitting this now on powerpc.  This patch never seemed to make
it upstream (drivers/tty/tty_ldisc.c hasn't been touched in 1 year).

Peter, can we take this patch as is, or do you have an updated version?

Mikey

> 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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