[PATCH 3.16 288/306] tty: Prevent ldisc drivers from re-using stale tty fields

Ben Hutchings
2017-02-15T23:13:41Z

From: Peter Hurley <peter@hurleysoftware.com>

commit dd42bf1197144ede075a9d4793123f7689e164bc upstream.

Line discipline drivers may mistakenly misuse ldisc-related fields
when initializing. For example, a failure to initialize tty->receive_room
in the N_GIGASET_M101 line discipline was recently found and fixed [1].
Now, the N_X25 line discipline has been discovered accessing the previous
line discipline's already-freed private data [2].

Harden the ldisc interface against misuse by initializing revelant
tty fields before instancing the new line discipline.

[1]
commit fd98e9419d8d622a4de91f76b306af6aa627aa9c
Author: Tilman Schmidt <tilman@imap.cc>
Date: Tue Jul 14 00:37:13 2015 +0200

isdn/gigaset: reset tty->receive_room when attaching ser_gigaset

[2] Report from Sasha Levin <sasha.levin@oracle.com>
[ 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)

Cc: Tilman Schmidt <tilman@imap.cc>
Cc: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/tty/tty_ldisc.c | 7 +++++++
1 file changed, 7 insertions(+)

--- a/drivers/tty/tty_ldisc.c
+++ b/drivers/tty/tty_ldisc.c
@@ -414,6 +414,10 @@ EXPORT_SYMBOL_GPL(tty_ldisc_flush);
* they are not on hot paths so a little discipline won't do
* any harm.
*
+ * The line discipline-related tty_struct fields are reset to
+ * prevent the ldisc driver from re-using stale information for
+ * the new ldisc instance.
+ *
* Locking: takes termios_rwsem
*/

@@ -422,6 +426,9 @@ static void tty_set_termios_ldisc(struct
down_write(&tty->termios_rwsem);
tty->termios.c_line = num;
up_write(&tty->termios_rwsem);
+
+ tty->disc_data = NULL;
+ tty->receive_room = 0;
}

/**