Re: [PATCH] serial: flush ldisc after hangup

From: Peter Hurley
Date: Tue Mar 01 2016 - 13:17:15 EST


Hi Josef,

On 03/01/2016 10:02 AM, Josef Bacik wrote:
> We hit a panic pretty consistently in production that looked like this
>
> PID: 461061 TASK: ffff880203f8bc00 CPU: 2 COMMAND: "kworker/u8:2"
> #0 [ffff88015834b940] machine_kexec at ffffffff8103c1c5
> #1 [ffff88015834b990] crash_kexec at ffffffff810cd448
> #2 [ffff88015834ba60] oops_end at ffffffff81006478
> #3 [ffff88015834ba90] no_context at ffffffff818c5262
> #4 [ffff88015834baf0] __bad_area_nosemaphore at ffffffff818c545a
> #5 [ffff88015834bb40] bad_area_nosemaphore at ffffffff818c548c
> #6 [ffff88015834bb50] __do_page_fault at ffffffff81045ad5
> #7 [ffff88015834bbc0] do_page_fault at ffffffff81045efc
> #8 [ffff88015834bbd0] page_fault at ffffffff818d6b82
> [exception RIP: __uart_start+0x1a]
> RIP: ffffffff8152f30a RSP: ffff88015834bc80 RFLAGS: 00010046
> RAX: 0000000000000000 RBX: ffffffff822e9920 RCX: 0000000000000036
> RDX: 0000000000003636 RSI: 00000000000000fe RDI: ffffffff822e9920
> RBP: ffff88015834bca8 R8: 0000000000000000 R9: 00000000ffffffff
> R10: ffff8802546f0d20 R11: 0000000000000000 R12: ffff880254712400
> R13: 0000000000000286 R14: 00000000000000fe R15: ffff880254712400
> ORIG_RAX: ffffffffffffffff CS: 0010 SS: 0018
> #9 [ffff88015834bc80] uart_start at ffffffff8152fbf2

Thanks for the report, but where's the rest of the stack trace?

> It was a NULL pointer dereference, the state->port.tty was NULL so when we go to
> check tty->stopped in uart_tx_stopped() we panic. Looking at the other CPU's we
> were in the middle of uart_open(), and the core actually had a valid pointer in
> state->port.tty, which points to a race between either close or hangup (the only
> two places that set state->port.tty to NULL) and open. Close already flushes
> the ldisc but hangup does not, which means we could have some characters in the
> receive buffer in between the hangup and the open, and we end up in this
> situation.

Yeah, the race is that the ldisc should not be attempting i/o to
the driver at all. This problem is fixed in -next already, but in the
tty core rather than in each individual tty driver.

Regards,
Peter Hurley

> Signed-off-by: Josef Bacik <jbacik@xxxxxx>
> ---
> drivers/tty/serial/serial_core.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index b1f54ab..93b7a53 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -1527,6 +1527,7 @@ static void uart_hangup(struct tty_struct *tty)
> wake_up_interruptible(&port->delta_msr_wait);
> }
> mutex_unlock(&port->mutex);
> + tty_ldisc_flush(tty);
> }
>
> static void uart_port_shutdown(struct tty_port *port)
>