On 2018/07/25 17:06, Greg KH wrote: > On Wed, Jul 25, 2018 at 12:22:16AM +0900, Tetsuo Handa wrote: >> >From 118c64e86641a97d44dec39e313a95b12d9bc3b2 Mon Sep 17 00:00:00 2001 >> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> >> Date: Wed, 25 Jul 2018 00:15:18 +0900 >> Subject: [PATCH v2] n_tty: Protect tty->disc_data using refcount. >> >> syzbot is reporting NULL pointer dereference at n_tty_set_termios() [1]. >> This is because ioctl(TIOCVHANGUP) versus ioctl(TCSETS) can race. >> >> Since we don't want to introduce new locking dependency, this patch >> converts "struct n_tty_data *ldata = tty->disc_data;" in individual >> function into a function argument which follows "struct tty *", and >> holds tty->disc_data at each "struct tty_ldisc_ops" hook using refcount >> in order to ensure that memory which contains "struct n_tty_data" will >> not be released while processing individual function. >> >> [1] https://syzkaller.appspot.com/bug?id=1e850009fca0b64ce49dc16499bda4f7de0ab1a5 >> >> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> >> --- >> drivers/tty/n_tty.c | 511 ++++++++++++++++++++++++++++++++-------------------- >> 1 file changed, 314 insertions(+), 197 deletions(-) > > What changed from v1? I haven't had the chance to review your first > patch, sorry, it's still in my queue. I was hoping that someone else > would help out with that as well :) > Just added a check in case I overlooked a subtle race condition that n_tty_close() is _somehow_ called twice. 25,26c20,21 < 1 file changed, 308 insertions(+), 195 deletions(-) > 1 file changed, 314 insertions(+), 197 deletions(-) 872,873c867,869 < struct n_tty_data *ldata = tty->disc_data; > - struct n_tty_data *ldata = tty->disc_data; > + struct n_tty_data *ldata = xchg(&tty->disc_data, NULL); > 878,879c874,880 < tty->disc_data = NULL; < + put_n_tty(ldata); > - tty->disc_data = NULL; > + /* > + * The xchg() above and this NULL test are rather paranoid checks. > + * Caller should not allow calling close() twice. > + */ > + if (ldata) > + put_n_tty(ldata); 1194c1195 < + if (!ldata) /* What value is most appropriate for this case? */ > + if (!ldata) If n_tty_close() is _somehow_ called twice, v1 patch will trigger NULL pointer dereference. If you are sure that n_tty_close() is never called twice, you can ignore v2.
