[4.2.y-ckt stable] Patch "wan/x25: Fix use-after-free in x25_asy_open_tty()" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    wan/x25: Fix use-after-free in x25_asy_open_tty()

to the linux-4.2.y-queue branch of the 4.2.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-4.2.y-queue

This patch is scheduled to be released in version 4.2.8-ckt2.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 4.2.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

---8<------------------------------------------------------------

>From e30c880eb93722108735ce45169d25cf3f03f631 Mon Sep 17 00:00:00 2001
From: Peter Hurley <peter at hurleysoftware.com>
Date: Fri, 27 Nov 2015 14:18:39 -0500
Subject: wan/x25: Fix use-after-free in x25_asy_open_tty()

commit ee9159ddce14bc1dec9435ae4e3bd3153e783706 upstream.

The N_X25 line discipline may access the previous line discipline's closed
and already-freed private data on open [1].

The tty->disc_data field _never_ refers to valid data on entry to the
line discipline's open() method. Rather, the ldisc is expected to
initialize that field for its own use for the lifetime of the instance
(ie. from open() to close() only).

[1]
    [  634.336761] ==================================================================
    [  634.338226] BUG: KASAN: use-after-free in x25_asy_open_tty+0x13d/0x490 at addr ffff8800a743efd0
    [  634.339558] Read of size 4 by task syzkaller_execu/8981
    [  634.340359] =============================================================================
    [  634.341598] BUG kmalloc-512 (Not tainted): kasan: bad access detected
    ...
    [  634.405018] Call Trace:
    [  634.405277] dump_stack (lib/dump_stack.c:52)
    [  634.405775] print_trailer (mm/slub.c:655)
    [  634.406361] object_err (mm/slub.c:662)
    [  634.406824] kasan_report_error (mm/kasan/report.c:138 mm/kasan/report.c:236)
    [  634.409581] __asan_report_load4_noabort (mm/kasan/report.c:279)
    [  634.411355] x25_asy_open_tty (drivers/net/wan/x25_asy.c:559 (discriminator 1))
    [  634.413997] tty_ldisc_open.isra.2 (drivers/tty/tty_ldisc.c:447)
    [  634.414549] tty_set_ldisc (drivers/tty/tty_ldisc.c:567)
    [  634.415057] tty_ioctl (drivers/tty/tty_io.c:2646 drivers/tty/tty_io.c:2879)
    [  634.423524] do_vfs_ioctl (fs/ioctl.c:43 fs/ioctl.c:607)
    [  634.427491] SyS_ioctl (fs/ioctl.c:622 fs/ioctl.c:613)
    [  634.427945] entry_SYSCALL_64_fastpath (arch/x86/entry/entry_64.S:188)

Reported-and-tested-by: Sasha Levin <sasha.levin at oracle.com>
Signed-off-by: Peter Hurley <peter at hurleysoftware.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 drivers/net/wan/x25_asy.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/drivers/net/wan/x25_asy.c b/drivers/net/wan/x25_asy.c
index 5c47b01..cd39025 100644
--- a/drivers/net/wan/x25_asy.c
+++ b/drivers/net/wan/x25_asy.c
@@ -549,16 +549,12 @@ static void x25_asy_receive_buf(struct tty_struct *tty,

 static int x25_asy_open_tty(struct tty_struct *tty)
 {
-	struct x25_asy *sl = tty->disc_data;
+	struct x25_asy *sl;
 	int err;

 	if (tty->ops->write == NULL)
 		return -EOPNOTSUPP;

-	/* First make sure we're not already connected. */
-	if (sl && sl->magic == X25_ASY_MAGIC)
-		return -EEXIST;
-
 	/* OK.  Find a free X.25 channel to use. */
 	sl = x25_asy_alloc();
 	if (sl == NULL)
--
1.9.1