[PATCH v2] tty: Fix data race in tty_insert_flip_string_fixed_flag

From: DaeRyong Jeong
Date: Mon Apr 30 2018 - 11:27:16 EST

Unlike normal serials, in pty layer, there is no guarantee that multiple
threads don't insert input characters at the same time. If it is happened,
tty_insert_flip_string_fixed_flag can be executed concurrently. This can
lead slab out-of-bounds write in tty_insert_flip_string_fixed_flag.

Call sequences are as follows.
n_tty_ioctl_helper n_tty_ioctl_helper
__start_tty tty_send_xchar
tty_wakeup pty_write
n_hdlc_tty_wakeup tty_insert_flip_string
n_hdlc_send_frames tty_insert_flip_string_fixed_flag

To fix the race, acquire port->lock in pty_write() before it inserts input
characters to tty buffer. It prevents multiple threads from inserting
input characters concurrently.

The crash log is as follows:
BUG: KASAN: slab-out-of-bounds in tty_insert_flip_string_fixed_flag+0xb5/
0x130 drivers/tty/tty_buffer.c:316 at addr ffff880114fcc121
Write of size 1792 by task syz-executor0/30017
CPU: 1 PID: 30017 Comm: syz-executor0 Not tainted 4.8.0 #1
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.8.2-0-g33fbe13 by qemu-project.org 04/01/2014
0000000000000000 ffff88011638f888 ffffffff81694cc3 ffff88007d802140
ffff880114fcb300 ffff880114fcc300 ffff880114fcb300 ffff88011638f8b0
ffffffff8130075c ffff88011638f940 ffff88007d802140 ffff880194fcc121
Call Trace:
__dump_stack lib/dump_stack.c:15 [inline]
dump_stack+0xb3/0x110 lib/dump_stack.c:51
kasan_object_err+0x1c/0x70 mm/kasan/report.c:156
print_address_description mm/kasan/report.c:194 [inline]
kasan_report_error+0x1f7/0x4e0 mm/kasan/report.c:283
kasan_report+0x36/0x40 mm/kasan/report.c:303
check_memory_region_inline mm/kasan/kasan.c:292 [inline]
check_memory_region+0x13e/0x1a0 mm/kasan/kasan.c:299
memcpy+0x37/0x50 mm/kasan/kasan.c:335
tty_insert_flip_string_fixed_flag+0xb5/0x130 drivers/tty/tty_buffer.c:316
tty_insert_flip_string include/linux/tty_flip.h:35 [inline]
pty_write+0x7f/0xc0 drivers/tty/pty.c:115
n_hdlc_send_frames+0x1d4/0x3b0 drivers/tty/n_hdlc.c:419
n_hdlc_tty_wakeup+0x73/0xa0 drivers/tty/n_hdlc.c:496
tty_wakeup+0x92/0xb0 drivers/tty/tty_io.c:601
__start_tty.part.26+0x66/0x70 drivers/tty/tty_io.c:1018
__start_tty+0x34/0x40 drivers/tty/tty_io.c:1013
n_tty_ioctl_helper+0x146/0x1e0 drivers/tty/tty_ioctl.c:1138
n_hdlc_tty_ioctl+0xb3/0x2b0 drivers/tty/n_hdlc.c:794
tty_ioctl+0xa85/0x16d0 drivers/tty/tty_io.c:2992
vfs_ioctl fs/ioctl.c:43 [inline]
do_vfs_ioctl+0x13e/0xba0 fs/ioctl.c:679
SYSC_ioctl fs/ioctl.c:694 [inline]
SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685

Signed-off-by: DaeRyong Jeong <threeearcat@xxxxxxxxx>
drivers/tty/pty.c | 3 +++
1 file changed, 3 insertions(+)

diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index 364c0e9..0c7ec27 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -110,16 +110,19 @@ static void pty_unthrottle(struct tty_struct *tty)
static int pty_write(struct tty_struct *tty, const unsigned char *buf, int c)
struct tty_struct *to = tty->link;
+ unsigned long flags;

if (tty->stopped)
return 0;

if (c > 0) {
+ spin_lock_irqsave(&to->port->lock, flags);
/* Stuff the data into the input queue of the other end */
c = tty_insert_flip_string(to->port, buf, c);
/* And shovel */
if (c)
+ spin_unlock_irqrestore(&to->port->lock, flags);
return c;