kernel-packages team mailing list archive

[John Johansen <john.johansen@xxxxxxxxxxxxx>]

** Description changed:

  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
+ 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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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-fsl-imx51” package in Ubuntu:
  New
Status in “linux-lts-quantal” package in Ubuntu:
  New
Status in “linux-lts-raring” package in Ubuntu:
  New
Status in “linux-lts-saucy” package in Ubuntu:
  New
Status in “linux-mvl-dove” package in Ubuntu:
  New
Status in “linux-ti-omap4” 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

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