Re: [PATCH] kernel panic, pty.c: remove direct call to tty_wakupin pty_write

[Andre Naujoks]

On 02.07.2013 11:39, schrieb Dean Jenkins:
> On 01/07/13 15:49, Andre Naujoks wrote:
>> Hello.
>>
>> This patch removes the direct call to tty_wakeup in pty_write. I have
>> not noticed any drawbacks with this but I am not familiar with the pty
>> driver at all. I think what happens is a recursive loop,
>> write_wakeup->write->write_wakeup ...
> Indeed there is a recursive loop that I have witnessed whilst using SLIP
> over USB HID.
> 
> In the failure case for SLIP, xleft remains positive and recursion
> causes a stack overflow as xleft is not updated during the recursion:
> sl_encaps()-->pty_write()-->tty_wakeup()-->slip_write_wakeup()-->pty_write()-->tty_wakeup()-->slip_write_wakeup()
> etc.

The funny thing about the SLIP driver is, that I could not reproduce
this particular issue with it. The SLIP driver would just hang after a
second of high load but never crash the kernel for me. It may be,
because I used a network connection with socat, which is fast enough for
the data to go through after a few recursions.

> 
> The underlying issue being pty_write() calling tty_wakeup() within the
> initial write thread. Note that the TTY wakeup event uses tty_wakeup()
> as a callback to request more data.
>> The documentation for the tty interface forbids this direct call:
>>
>> (from Documentation/serial/tty.txt)
>> write_wakeup()  - May be called at any point between open and close.
>>        The TTY_DO_WRITE_WAKEUP flag indicates if a call
>>        is needed but always races versus calls. Thus the
>>        ldisc must be careful about setting order and to
>>        handle unexpected calls. Must not sleep.
>>
>>        The driver is forbidden from calling this directly
>>        from the ->write call from the ldisc as the ldisc
>>        is permitted to call the driver write method from
>>        this function. In such a situation defer it.
> I also saw that documentation and the code seems to be breaking that
> description.
> 
> It is possible to mitigate against
> pty-write-->tty_wakeup()-->slip_write_wakeup() by clearing and setting
> TTY_DO_WRITE_WAKEUP at the "correct" time in the layers bound to
> PTY/TTY. Indeed, I modified SLIP to do that and sent patches to the
> linux-netdev mailing list although I am not aware of any progress in
> having those patches accepted.

See below, why I don't think, that that is a good idea.

> 
> Perhaps this mitigation could be applied to your scenario ?
> 
> Eventually, your patch could be used when all protocols have mitigration
> in place.
> 
>>
>>
>> The direct call caused a reproducable kernel panic (see bottom of this
>> mail) for me with the following setup:
>>
>> - using can-utils from git://gitorious.org/linux-can/can-utils.git
>>    slcan_attach and cangen are used
>>
>> - create a network link between two serial CAN interfaces with:
>>    $ socat PTY,link=/tmp/slcan0,raw TCP4-LISTEN:50000 &
>>    $ socat TCP4:localhost:50000 PTY,link=/tmp/slcan1,raw &
>>    $ slcan_attach /tmp/slcan0
>>    $ slcan_attach /tmp/slcan1
>>    $ ip link set slcan0 up
>>    $ ip link set slcan1 up
>>
>> - produce a kernel panic by overloading the CAN interfaces:
>>    $ cangen slcan0 -g0
>>
>>
>> Please keep me in CC. I am not subscribed to the list.
>> If I can provide any more information, I will be glad to do so.
>>
>> This is the patch. It applies to the current linux master branch:
>>
>>
>>  From 9f67139bebb938026406a66c1411e0b50628a238 Mon Sep 17 00:00:00 2001
>> From: Andre Naujoks <nautsch2@xxxxxxxxxxxxxx>
>> Date: Mon, 1 Jul 2013 15:45:13 +0200
>> Subject: [PATCH 1/2] remove direct call to tty_wakeup in pty_write.
>>
>> Signed-off-by: Andre Naujoks <nautsch2@xxxxxxxxxxxxxx>
>> ---
>>   drivers/tty/pty.c | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
>> index abfd990..5dcb782 100644
>> --- a/drivers/tty/pty.c
>> +++ b/drivers/tty/pty.c
>> @@ -127,7 +127,6 @@ static int pty_write(struct tty_struct *tty, const
>> unsigned char *buf, int c)
>>           /* And shovel */
>>           if (c) {
>>               tty_flip_buffer_push(to->port);
>> -            tty_wakeup(tty);
>>           }
>>       }
>>       return c;
> I agree that this looks to be a simple remedy to the issue. However,
> this code has existed in this state for many years so there is a risk
> that some applications rely on this "feature" in order to work. For
> example, SLIP uses this "feature" although I am not certain that the
> existing SLIP code would break if your patch were applied. The TTY
> wakeup event should take care of completing the transmission (in theory).

The SLIP driver doesn't break. It is actually more stable and does not
hang for me any more with high loads.

For example this hangs for me after a second or two over a
slip-pty-socat connection without the fix:

machineA$ netcat -l -p 60000
machineB$ cat /dev/zero | netcat <machineA-ip> 60000

With the fix, it does not hang any more.


I am currently using this patch and it "just works". I agree, that this
should probably be tested by some more people which are likely to be
affected by this.

I'd really like a comment from Greg Kroah-Hartman or Jiri Slaby on this,
because they are the maintainers for the pty driver and probably know
about any implications of this.

> 
> I think a lower risk approach would be to modify the upper layers in the
> protocol write function to clear the TTY_DO_WRITE_WAKEUP flag before
> pty_write is called and set TTY_DO_WRITE_WAKEUP afterwards to allow the
> TTY wakeup event to complete the transmission. I did this in the SLIP

Does this cause a race? What if the callback is defered and we have not
re-set the flag yet, when it should be called?

The wakup function is called even with the direct call removed,
otherwise the slcan and the slip drivers would no longer work, it is
just defered as required by the documentation.

I think there is no way around fixing the pty driver. All workarounds I
could think of (including disabling the callback for the duration of the
write inside the callback) are somehow broken.

> sl_encaps() and slip_write_wakeup() functions. Note that the SLIP
> segmentation mechanism is also broken as it is possible to send
> truncated SLIP frames upon a failure to send all characters of the

Is that really a problem? The SLIP driver (and the slcan driver) stops
the netif queue before entering sl_encaps and wakes it up again, when
all data is transmitted (from the wakup call), doesn't it? So there
should be not further calls to sl_xmit while the queue is stopped. But I
might misunderstand the semantics of netif_stop_queue and
netif_wake_queue here.

Regards
  Andre

> frame. This leads to the recursive loop on the transmission of next SLIP
> frame because xleft remains positive (uninitialised) and this causes the
> kernel to crash catastrophically. The scheduler crashes due to the stack
> overflow overwriting the task data structures.
> 
> If anyone is interested, I could upload my SLIP patches to the
> linux-kernel mailing list.
> 
> Regards,
> Dean Jenkins
> Mentor Graphics

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