Hi Johan, >>> 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. I think for PTYs we should just fail setting the HCI line discipline. Fail early and just move on with life. Regards Marcel
