• Peter Hurley's avatar
    pty: Fix input race when closing · 1a48632f
    Peter Hurley authored
    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: 52bce7f8 ("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: default avatarAndy Whitcroft <apw@canonical.com>
    Reported-by: default avatarH.J. Lu <hjl.tools@gmail.com>
    Signed-off-by: default avatarPeter Hurley <peter@hurleysoftware.com>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    1a48632f
tty.txt 10.8 KB