n_tty: Fix n_tty_write crash when echoing in raw mode

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
present in kernels at least after
commit
d945cb9 (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
js: lock unconditionally
js: lock only the tty->ops->write
call

References: CVE-2014-0196
Reported-and-tested-by: Jiri Slaby
<jslaby@xxxxxxx>
Signed-off-by: Peter Hurley
<peter@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jiri Slaby
<jslaby@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc:
Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>
Cc:
<stable@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx>
[bwh: Backported to 3.2: output_lock is a
member of struct tty_struct]
Signed-off-by: Ben Hutchings
<ben@xxxxxxxxxxxxxxx>

Change-Id:
Ie00ddace4af1824c5e1ea56c3896896f9aae3092

idea is taken here -
https://github.com/rmbq/android_kernel_htc_msm8960/commit/2790d906eb92394a3366abb65adaa6d2efd2248f