linux-kernel - Re: [PATCH v2] tty: Fix WARNING in tty_set

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Message-ID: <7dc2f2b5-95f4-4809-7719-d214443966eb@kernel.org>
Date:   Fri, 1 Feb 2019 16:18:57 -0700
From:   shuah <shuah@...nel.org>
To:     Marcel Holtmann <marcel@...tmann.org>,
        Greg KH <gregkh@...uxfoundation.org>
Cc:     Johan Hedberg <johan.hedberg@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Bluez mailing list <linux-bluetooth@...r.kernel.org>,
        linux-kernel@...r.kernel.org, Johan Hovold <johan@...nel.org>,
        shuah <shuah@...nel.org>
Subject: Re: [PATCH v2] tty: Fix WARNING in tty_set_termios()

On 2/1/19 3:00 AM, Marcel Holtmann wrote:
> Hi Shuah,
> 
>> tty_set_termios() has the following WARN_ON which can be triggered with a
>> syscall to invoke TIOCSETD __NR_ioctl.
>>
>> WARN_ON(tty->driver->type == TTY_DRIVER_TYPE_PTY &&
>>                 tty->driver->subtype == PTY_TYPE_MASTER);
>> Reference: https://syzkaller.appspot.com/bug?id=2410d22f1d8e5984217329dd0884b01d99e3e48d
>>
>> The problem started with commit 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.
>>
>> Fix the problem by preventing setting the HCI line discipline for PTYs
>> from hci_uart_setup() and hci_uart_set_flow_control().
>>
>> The reproducer is used to reproduce the problem and verify the fix.
>>
>> Reported-by: syzbot+a950165cbb86bdd023a4@...kaller.appspotmail.com
>> Signed-off-by: Shuah Khan <shuah@...nel.org>
>> ---
>> drivers/bluetooth/hci_ldisc.c | 11 +++++++++++
>> 1 file changed, 11 insertions(+)
>>
>> diff --git a/drivers/bluetooth/hci_ldisc.c b/drivers/bluetooth/hci_ldisc.c
>> index fbf7b4df23ab..ce84ca91ca70 100644
>> --- a/drivers/bluetooth/hci_ldisc.c
>> +++ b/drivers/bluetooth/hci_ldisc.c
>> @@ -314,6 +314,11 @@ void hci_uart_set_flow_control(struct hci_uart *hu, bool enable)
>> 		return;
>> 	}
>>
>> +	/* don't set HCI line discipline on PTYs */
>> +	if (tty->driver->type == TTY_DRIVER_TYPE_PTY &&
>> +	    tty->driver->subtype == PTY_TYPE_MASTER)
>> +		return;
>> +
> 
> just do it once in hci_uart_tty_open. Actually we already check for ops->write and so lets just ensure all ops we ever call are present. If they are not, then bail out. I wouldn’t even bother with the TTY type. Wouldn’t that be a lot simpler?
> 

As you said, hci_uart_tty_open()is a good place to check it. I think
checking tty type is necessary. I couldn't find any missing ops I could
base the check on to prevent the ldisc set.

I have the patch that does the tty check in hci_uart_tty_open() tested
and ready for sending.

thanks,
-- Shuah