Please sign in to comment.
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 <firstname.lastname@example.org> Signed-off-by: Peter Hurley <email@example.com> Signed-off-by: Jiri Slaby <firstname.lastname@example.org> Cc: Linus Torvalds <email@example.com> Cc: Alan Cox <firstname.lastname@example.org> Cc: <email@example.com> Signed-off-by: Greg Kroah-Hartman <firstname.lastname@example.org>
- Loading branch information...