linux-kernel - Re: [PATCH v1] tty: serdev: serdev-ttyport: Fix use-after-free in ttyport_close() due to uninitialized serport->tty

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Message-ID: <38bf94e1-ebed-4d03-8ea0-4040009e8d31@quicinc.com>
Date: Wed, 14 May 2025 17:14:04 +0800
From: Xin Chen <quic_cxin@...cinc.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC: Rob Herring <robh@...nel.org>, Jiri Slaby <jirislaby@...nel.org>,
        <linux-serial@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <liulzhao@....qualcomm.com>, <quic_chejiang@...cinc.com>,
        <zaiyongc@....qualcomm.com>, <quic_zijuhu@...cinc.com>,
        <quic_mohamull@...cinc.com>,
        Panicker Harish <quic_pharish@...cinc.com>
Subject: Re: [PATCH v1] tty: serdev: serdev-ttyport: Fix use-after-free in
 ttyport_close() due to uninitialized serport->tty

On 5/8/2025 5:41 PM, Greg Kroah-Hartman wrote:
> On Thu, May 08, 2025 at 05:29:18PM +0800, Xin Chen wrote:
>>
>> On 4/30/2025 7:40 PM, Greg Kroah-Hartman wrote:
>>> On Wed, Apr 30, 2025 at 07:16:17PM +0800, Xin Chen wrote:
>>>> When ttyport_open() fails to initialize a tty device, serport->tty is not
>>>> --- a/drivers/tty/serdev/serdev-ttyport.c
>>>> +++ b/drivers/tty/serdev/serdev-ttyport.c
>>>> @@ -88,6 +88,10 @@ static void ttyport_write_flush(struct serdev_controller *ctrl)
>>>>  {
>>>>  	struct serport *serport = serdev_controller_get_drvdata(ctrl);
>>>>  	struct tty_struct *tty = serport->tty;
>>>> +	if (!tty) {
>>>> +		dev_err(&ctrl->dev, "tty is null\n");
>>>> +		return;
>>>> +	}
>>>
>>> What prevents tty from going NULL right after you just checked this?
>>
>> First sorry for reply so late for I have a long statutory holidays.
>> Maybe I don't get your point. From my side, there is nothing to prevent it.
>> Check here is to avoid code go on if tty is NULL.
> 
> Yes, but the problem is, serport->tty could change to be NULL right
> after you check it, so you have not removed the real race that can
> happen here.  There is no lock, so by adding this check you are only
> reducing the risk of the problem happening, not actually fixing the
> issue so that it will never happen.
> 
> Please fix it so that this can never happen.
> 

Actually I have never thought the race condition issue since the crash I met is
not caused by race condition. It's caused due to Bluetooth driver call
ttyport_close() after ttyport_open() failed. This two action happen one after
another in one thread and it seems impossible to have race condition. And with
my fix the crash doesn't happen again in several test of same case.

Let me introduce the complete process for you:
  1) hci_dev_open_sync()->
hci_dev_init_sync()->hci_dev_setup_sync()->hdev->setup()(hci_uart_setup)->qca_setup(),
here in qca_setup(), qca_read_soc_version() fails and goto out, then calls
serdev_device_close() to close tty normally. And then call serdev_device_open()
to retry.
  2) serdev_device_open() fails due to tty_init_dev() fails, then tty gets
released, which means this time the tty has been freed succesfully.
  3) Return back to upper func  hci_dev_open_sync(),
hdev->close()(hci_uart_close) is called. And hci_uart_close calls
hci_uart_flush() and serdev_device_close(). serdev_device_close() tries to close
tty again, it's calltrace is serdev_device_close()->ttyport_close()->tty_lock(),
tty_unlock(), tty_release_struct(). The four funcs hci_uart_flush(), tty_lock(),
tty_unlock(), tty_release_struct() read tty pointer's value, which is invalid
and causes crash.