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

[H.J. Lu]

On Thu, Apr 9, 2015 at 7:54 AM, Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx> # 3.19+
> Reported-by: Andy Whitcroft <apw@xxxxxxxxxxxxx>
> Reported-by: H.J. Lu <hjl.tools@xxxxxxxxx>
> Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
> ---
>  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


-- 
H.J.
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