This is a note to let you know that I've just added the patch titled
n_tty: Fix n_tty_write crash when echoing in raw mode
to the 3.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
n_tty-Fix-n_tty_write-crash-when-echoing-in-raw-mode.patch
and it can be found in the queue-3.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
From: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
Date: Sat, 3 May 2014 14:04:59 +0200
Subject: n_tty: Fix n_tty_write crash when echoing in raw mode
commit 4291086b1f081b869c6d79e5b7441633dc3ace00 upstream.
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@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>
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>
---
drivers/tty/n_tty.c | 2 ++
1 file changed, 2 insertions(+)
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1996,7 +1996,9 @@ static ssize_t n_tty_write(struct tty_st
tty->ops->flush_chars(tty);
} else {
while (nr > 0) {
+ mutex_lock(&tty->output_lock);
c = tty->ops->write(tty, b, nr);
+ mutex_unlock(&tty->output_lock);
if (c < 0) {
retval = c;
goto break_out;
Patches currently in stable-queue which might be from peter@xxxxxxxxxxxxxxxxxx are
queue-3.4/n_tty-Fix-n_tty_write-crash-when-echoing-in-raw-mode.patch
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