>From 34ba81cd3561c5fc6aff3041f3445d69f85b5155 Mon Sep 17 00:00:00 2001 From: Peter Hurley Date: Tue, 29 Apr 2014 12:38:36 -0400 Subject: [PATCH 1/1] 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 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 Reported-and-tested-by: Jiri Slaby Signed-off-by: Peter Hurley Signed-off-by: Jiri Slaby Cc: Alan Cox --- Hi security team, this is a fix for a potentially exploitable hole in the TTY layer. linux-distros ML has been informed already (to the best of my knowledge). We, at suse, would appreciate embargo till Mon May 5th. Thanks. drivers/tty/n_tty.c | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index 746ae80b972f..94101fc64e02 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -2353,10 +2353,18 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file, if (tty->ops->flush_chars) tty->ops->flush_chars(tty); } else { + struct n_tty_data *ldata = tty->disc_data; + bool lock; + + lock = L_ECHO(tty) || (ldata->icanon && L_ECHONL(tty)); + if (lock) + mutex_lock(&ldata->output_lock); while (nr > 0) { c = tty->ops->write(tty, b, nr); if (c < 0) { retval = c; + if (lock) + mutex_unlock(&ldata->output_lock); goto break_out; } if (!c) @@ -2364,6 +2372,8 @@ static ssize_t n_tty_write(struct tty_struct *tty, struct file *file, b += c; nr -= c; } + if (lock) + mutex_unlock(&ldata->output_lock); } if (!nr) break; -- 1.9.2