Re: [PATCH] pty: Fix input race when closing

[Peter Hurley <peter@hurleysoftware.com>]

On 04/09/2015 01:43 PM, H.J. Lu wrote:
> On Thu, Apr 9, 2015 at 7:54 AM, Peter Hurley <peter@hurleysoftware.com> wrote:
>> A read() from a pty master may mistakenly indicate EOF (errno == -EIO)
>> after the pty slave has closed, even though input data remains to be read.
>> For example,
>>
>>        pty slave       |        input worker        |    pty master
>>                        |                            |
>>                        |                            |   n_tty_read()
>> pty_write()            |                            |     input avail? no
>>   add data             |                            |     sleep
>>   schedule worker  --->|                            |     .
>>                        |---> flush_to_ldisc()       |     .
>> pty_close()            |       fill read buffer     |     .
>>   wait for worker      |       wakeup reader    --->|     .
>>                        |       read buffer full?    |---> input avail ? yes
>>                        |<---   yes - exit worker    |     copy 4096 bytes to user
>>   TTY_OTHER_CLOSED <---|                            |<--- kick worker
>>                        |                            |
>>
>>                                 **** New read() before worker starts ****
>>
>>                        |                            |   n_tty_read()
>>                        |                            |     input avail? no
>>                        |                            |     TTY_OTHER_CLOSED? yes
>>                        |                            |     return -EIO
>>
>> Several conditions are required to trigger this race:
>> 1. the ldisc read buffer must become full so the input worker exits
>> 2. the read() count parameter must be >= 4096 so the ldisc read buffer
>>    is empty
>> 3. the subsequent read() occurs before the kicked worker has processed
>>    more input
>>
>> However, the underlying cause of the race is that data is pipelined, while
>> tty state is not; ie., data already written by the pty slave end is not
>> yet visible to the pty master end, but state changes by the pty slave end
>> are visible to the pty master end immediately.
>>
>> Pipeline the TTY_OTHER_CLOSED state through input worker to the reader.
>> 1. Introduce TTY_OTHER_DONE which is set by the input worker when
>>    TTY_OTHER_CLOSED is set and either the input buffers are flushed or
>>    input processing has completed. Readers/polls are woken when
>>    TTY_OTHER_DONE is set.
>> 2. Reader/poll checks TTY_OTHER_DONE instead of TTY_OTHER_CLOSED.
>> 3. A new input worker is started from pty_close() after setting
>>    TTY_OTHER_CLOSED, which ensures the TTY_OTHER_DONE state will be
>>    set if the last input worker is already finished (or just about to
>>    exit).
>>
>> Remove tty_flush_to_ldisc(); no in-tree callers.
>>
>> Fixes: 52bce7f8d4fc ("pty, n_tty: Simplify input processing on final close")
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=96311
>> BugLink: http://bugs.launchpad.net/bugs/1429756
>> Cc: <stable@vger.kernel.org> # 3.19+
>> Reported-by: Andy Whitcroft <apw@canonical.com>
>> Reported-by: H.J. Lu <hjl.tools@gmail.com>
>> Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
>> ---
>>  Documentation/serial/tty.txt |  3 +++
>>  drivers/tty/n_hdlc.c         |  4 ++--
>>  drivers/tty/n_tty.c          |  4 ++--
>>  drivers/tty/pty.c            |  3 +--
>>  drivers/tty/tty_buffer.c     | 25 +++++++++++--------------
>>  include/linux/tty.h          |  2 +-
>>  6 files changed, 20 insertions(+), 21 deletions(-)
>>
> 
> I tried it on 3.19.3 and it doesn't work with the testcase in
> 
> https://bugzilla.kernel.org/show_bug.cgi?id=96311

Can you test this patch on top of mainline? There's a couple of
fixes in 4.0-rc that specifically address weakly-ordered CPUs.

Regards,
Peter Hurley