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[John Johansen <john.johansen@xxxxxxxxxxxxx>]

** Information type changed from Private Security to Public Security

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https://bugs.launchpad.net/bugs/1314762

Title:
  n_tty_write crash when echoing in raw mode

Status in “linux” package in Ubuntu:
  New
Status in “linux-armadaxp” package in Ubuntu:
  New
Status in “linux-ec2” package in Ubuntu:
  New
Status in “linux-lts-quantal” package in Ubuntu:
  New
Status in “linux-lts-saucy” package in Ubuntu:
  New
Status in “linux-mvl-dove” package in Ubuntu:
  New

Bug description:
  The tty atomic_write_lock does not provide an exclusion guarantee for
  the tty driver if the termios settings are LECHO & !OPOST.  And since
  it is unexpected and not allowed to call TTY buffer helpers like
  tty_insert_flip_string concurrently, this may lead to crashes when
  concurrect writers call pty_write. In that case the following two
  writers:
  * the ECHOing from a workqueue and
  * pty_write from the process
  race and can overflow the corresponding TTY buffer like follows.

  If we look into tty_insert_flip_string_fixed_flag, there is:
    int space = __tty_buffer_request_room(port, goal, flags);
    struct tty_buffer *tb = port->buf.tail;
    ...
    memcpy(char_buf_ptr(tb, tb->used), chars, space);
    ...
    tb->used += space;

  so the race of the two can result in something like this:
                A                                B
  __tty_buffer_request_room
                                    __tty_buffer_request_room
  memcpy(buf(tb->used), ...)
  tb->used += space;
                                    memcpy(buf(tb->used), ...) ->BOOM

  B's memcpy is past the tty_buffer due to the previous A's tb->used
  increment.

  Since the N_TTY line discipline input processing can output
  concurrently with a tty write, obtain the N_TTY ldisc output_lock to
  serialize echo output with normal tty writes.  This ensures the tty
  buffer helper tty_insert_flip_string is not called concurrently and
  everything is fine.

  Note that this is nicely reproducible by an ordinary user using
  forkpty and some setup around that (raw termios + ECHO). And it is
  exploitable in kernels at least after commit
  d945cb9cce20ac7143c2de8d88b187f62db99bdc (pty: Rework the pty layer to
  use the normal buffering logic) in 2.6.31-rc3.

  js: add more info to the commit log
  js: switch to bool

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