From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CE008C282DB for ; Thu, 31 Jan 2019 15:18:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id AB21B20863 for ; Thu, 31 Jan 2019 15:18:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731265AbfAaPSg convert rfc822-to-8bit (ORCPT ); Thu, 31 Jan 2019 10:18:36 -0500 Received: from coyote.holtmann.net ([212.227.132.17]:37725 "EHLO mail.holtmann.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726239AbfAaPSg (ORCPT ); Thu, 31 Jan 2019 10:18:36 -0500 Received: from marcel-macpro.fritz.box (p4FF9FD60.dip0.t-ipconnect.de [79.249.253.96]) by mail.holtmann.org (Postfix) with ESMTPSA id A490ECEE02; Thu, 31 Jan 2019 16:26:22 +0100 (CET) Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 12.2 \(3445.102.3\)) Subject: Re: [PATCH] tty: Fix WARNING in tty_set_termios From: Marcel Holtmann In-Reply-To: <20190130103227.GR3691@localhost> Date: Thu, 31 Jan 2019 16:18:33 +0100 Cc: shuah , Al Viro , "open list:NFC SUBSYSTEM" , chris@the-brannons.com, devel@driverdev.osuosl.org, Rob Herring , Samuel Ortiz , "open list:SERIAL DRIVERS" , Jiri Slaby , santhameena13@gmail.com, kirk@reisers.ca, Johan Hedberg , Arnd Bergmann , samuel.thibault@ens-lyon.org, m.maya.nakamura@gmail.com, zhongjiang@huawei.com, Greg KH , speakup@linux-speakup.org, Linux Kernel Mailing List , Bluez mailing list , netdev , nishka.dasgupta_ug18@ashoka.edu.in, "David S. Miller" Content-Transfer-Encoding: 8BIT Message-Id: References: <20190125232905.21727-1-shuah@kernel.org> <20190126041416.GF2217@ZenIV.linux.org.uk> <20190130103227.GR3691@localhost> To: Johan Hovold X-Mailer: Apple Mail (2.3445.102.3) Sender: linux-bluetooth-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-bluetooth@vger.kernel.org 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