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Date: Mon, 5 May 2014 12:08:11 +0200
From: Marcus Meissner <>
To: OSS Security List <>
Subject: CVE-2014-0196: Linux kernel pty layer race condition memory


SUSE customer Ericsson reported a kernel crash to us which turned out
to be a race condition in the PTY write buffer handling.

When two processes/threads write to the same pty, the buffer end could
be overwritten and so memory corruption into adjacent buffers could lead
to crashes / code execution.

Jiri Slaby and Peter Hurley localized and fixed this problem.

CVE-2014-0196 has been assigned to this issue.

Jiri thinks this was introduced during 2.6.31 development by
d945cb9cce20ac7143c2de8d88b187f62db99bdc (pty: Rework the pty
layer to use the normal buffering logic) in 2.6.31-rc3. Until then, pty
was writing directly to a line discipline without using buffers.

Patch is also attached.

Ciao, Marcus

>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
* 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
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

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

We, at suse, would appreciate embargo till Mon May 5th.


 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)
 		} 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)

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