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

[Xin Chen <quic_cxin@quicinc.com>]

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.
> 
> And why print out that message, what can userspace do with it?
> 

I add the print just ref to code in other place. This can't be used by
userspace, but it can be used in DMesg log when system crashes.
>>
>>  	tty_driver_flush_buffer(tty);
>>  }
>> @@ -108,8 +112,10 @@ static int ttyport_open(struct serdev_controller *ctrl)
>>  	int ret;
>>
>>  	tty = tty_init_dev(serport->tty_drv, serport->tty_idx);
>> -	if (IS_ERR(tty))
>> +	if (IS_ERR(tty)) {
>> +		serport->tty = NULL;
>>  		return PTR_ERR(tty);
>> +	}
>>  	serport->tty = tty;
>>
>>  	if (!tty->ops->open || !tty->ops->close) {
>> @@ -156,6 +162,11 @@ static void ttyport_close(struct serdev_controller *ctrl)
>>
>>  	clear_bit(SERPORT_ACTIVE, &serport->flags);
>>
>> +	if (!tty) {
>> +		dev_err(&ctrl->dev, "tty is null\n");
>> +		return;
>> +	}
> 
> Again, what prevents it from changing right after you just checked it?

Same with above, there is nothing prevent it. Check here is to avoid code go on
if tty is NULL.
The check is for changes in ttyport_open(). In my project, it's possible that
ttyport_close() and ttyport_write_flush() get called after ttyport_open()
failed, at which time tty is invalid.

thanks,
Xin