diff options
author | Peter Hurley <peter@hurleysoftware.com> | 2014-05-03 12:04:59 (GMT) |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2014-05-03 22:13:05 (GMT) |
commit | 4291086b1f081b869c6d79e5b7441633dc3ace00 (patch) | |
tree | c37b5ba253d23f6ece2ce8b95b94a909792666f4 /drivers/tty | |
parent | b790f210fe8423eff881b2a8a93ba5dbc45534d0 (diff) | |
download | linux-4291086b1f081b869c6d79e5b7441633dc3ace00.tar.xz |
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
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
js: lock unconditionally
js: lock only the tty->ops->write call
References: CVE-2014-0196
Reported-and-tested-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'drivers/tty')
-rw-r--r-- | drivers/tty/n_tty.c | 4 |
1 files changed, 4 insertions, 0 deletions
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c index 41fe8a0..fe9d129 100644 --- a/drivers/tty/n_tty.c +++ b/drivers/tty/n_tty.c @@ -2353,8 +2353,12 @@ 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; + while (nr > 0) { + mutex_lock(&ldata->output_lock); c = tty->ops->write(tty, b, nr); + mutex_unlock(&ldata->output_lock); if (c < 0) { retval = c; goto break_out; |