On Mon, Jan 28, 2019 at 02:29:22PM -0700, shuah wrote: > On 1/25/19 9:14 PM, Al Viro wrote: > > On Fri, Jan 25, 2019 at 04:29:05PM -0700, Shuah Khan wrote: > >> tty_set_termios() has the following WARMN_ON which can be triggered with a > >> syscall to invoke TIOCGETD __NR_ioctl. You meant TIOCSETD here, and in fact its the call which sets the uart protocol that triggers the warning. > >> WARN_ON(tty->driver->type == TTY_DRIVER_TYPE_PTY && > >> tty->driver->subtype == PTY_TYPE_MASTER); > >> Reference: https://syzkaller.appspot.com/bug?id=2410d22f1d8e5984217329dd0884b01d99e3e48d > >> > >> A simple change would have been to print error message instead of WARN_ON. > >> However, the callers assume that tty_set_termios() always returns 0 and > >> don't check return value. The complete solution is fixing all the callers > >> to check error and bail out to fix the WARN_ON. > >> > >> This fix changes tty_set_termios() to return error and all the callers > >> to check error and bail out. The reproducer is used to reproduce the > >> problem and verify the fix. > > > >> --- a/drivers/bluetooth/hci_ldisc.c > >> +++ b/drivers/bluetooth/hci_ldisc.c > >> @@ -321,6 +321,8 @@ void hci_uart_set_flow_control(struct hci_uart *hu, bool enable) > >> status = tty_set_termios(tty, &ktermios); > >> BT_DBG("Disabling hardware flow control: %s", > >> status ? "failed" : "success"); > >> + if (status) > >> + return; > > > > Can that ldisc end up set on pty master? And does it make any sense there? > > The initial objective of the patch is to prevent the WARN_ON by making > the change to return error instead of WARN_ON. However, without changes > to places that don't check the return and keep making progress, there > will be secondary problems. > > Without this change to return here, instead of WARN_ON, it will fail > with the following NULL pointer dereference at the next thing > hci_uart_set_flow_control() attempts. > > status = tty->driver->ops->tiocmget(tty); > > kernel: [10140.649783] BUG: unable to handle kernel NULL pointer That's a separate issue, which is being fixed: https://lkml.kernel.org/r/20190130095938.GP3691@localhost > > IOW, I don't believe that this patch makes any sense. If anything, > > we need to prevent unconditional tty_set_termios() on the path > > that *does* lead to calling it for pty. > > I don't think preventing unconditional tty_set_termios() is enough to > prevent secondary problems such as the one above. > > For example, the following call chain leads to the WARN_ON that was > reported. Even if void hci_uart_set_baudrate() prevents the very first > tty_set_termios() call, its caller hci_uart_setup() continues with > more tty setup. It goes ahead to call driver setup callback. The > driver callback goes on to do more setup calling tty_set_termios(). > > WARN_ON call path: > hci_uart_set_baudrate+0x1cc/0x250 drivers/bluetooth/hci_ldisc.c:378 > hci_uart_setup+0xa2/0x490 drivers/bluetooth/hci_ldisc.c:401 > hci_dev_do_open+0x6b1/0x1920 net/bluetooth/hci_core.c:1423 > > Once this WARN_ON is changed to return error, the following > happens, when hci_uart_setup() does driver setup callback. > > kernel: [10140.649836] mrvl_setup+0x17/0x80 [hci_uart] > kernel: [10140.649840] hci_uart_setup+0x56/0x160 [hci_uart] > kernel: [10140.649850] hci_dev_do_open+0xe6/0x630 [bluetooth] > kernel: [10140.649860] hci_power_on+0x52/0x220 [bluetooth] > > I think continuing to catch the invalid condition in tty_set_termios() > and preventing progress by checking return value is a straight forward > change to avoid secondary problems, and it might be difficult to catch > all the cases where it could fail. I agree with Al that this change doesn't make much sense. The WARN_ON is there to catch any bugs leading to the termios being changed for a master side pty. Those should bugs should be fixed, and not worked around in order to silence a WARN_ON. The problem started with 7721383f4199 ("Bluetooth: hci_uart: Support operational speed during setup") which introduced a new way for how tty_set_termios() could end up being called for a master pty. As Al hinted at, setting these ldiscs for a master pty really makes no sense and perhaps that is what we should prevent unless simply making sure they do not call tty_set_termios() is sufficient for the time being. Finally, note that serdev never operates on a pty, and that this is only an issue for (the three) line disciplines. Johan
