Subject: Re: [PATCH] tty: hvc: Fix data abort due to race in hvc_open

From: [email-masked]
Date: Sat, 09 May 2020 18:30:56 -0700

On 2020-05-06 02:48, Greg KH wrote:
> On Mon, Apr 27, 2020 at 08:26:01PM -0700, Raghavendra Rao Ananta wrote:
>> Potentially, hvc_open() can be called in parallel when two tasks calls
>> open() on /dev/hvcX. In such a scenario, if the 
>> hp->ops->notifier_add()
>> callback in the function fails, where it sets the tty->driver_data to
>> NULL, the parallel hvc_open() can see this NULL and cause a memory 
>> abort.
>> Hence, serialize hvc_open and check if tty->private_data is NULL 
>> before
>> proceeding ahead.
>> 
>> The issue can be easily reproduced by launching two tasks 
>> simultaneously
>> that does nothing but open() and close() on /dev/hvcX.
>> For example:
>> $ ./simple_open_close /dev/hvc0 & ./simple_open_close /dev/hvc0 &
>> 
>> Signed-off-by: Raghavendra Rao Ananta <[email-masked]>
>> ---
>>  drivers/tty/hvc/hvc_console.c | 16 ++++++++++++++--
>>  1 file changed, 14 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/tty/hvc/hvc_console.c 
>> b/drivers/tty/hvc/hvc_console.c
>> index 436cc51c92c3..ebe26fe5ac09 100644
>> --- a/drivers/tty/hvc/hvc_console.c
>> +++ b/drivers/tty/hvc/hvc_console.c
>> @@ -75,6 +75,8 @@ static LIST_HEAD(hvc_structs);
>>   */
>>  static DEFINE_MUTEX(hvc_structs_mutex);
>> 
>> +/* Mutex to serialize hvc_open */
>> +static DEFINE_MUTEX(hvc_open_mutex);
>>  /*
>>   * This value is used to assign a tty->index value to a hvc_struct 
>> based
>>   * upon order of exposure via hvc_probe(), when we can not match it 
>> to
>> @@ -346,16 +348,24 @@ static int hvc_install(struct tty_driver 
>> *driver, struct tty_struct *tty)
>>   */
>>  static int hvc_open(struct tty_struct *tty, struct file * filp)
>>  {
>> -	struct hvc_struct *hp = tty->driver_data;
>> +	struct hvc_struct *hp;
>>  	unsigned long flags;
>>  	int rc = 0;
>> 
>> +	mutex_lock(&hvc_open_mutex);
>> +
>> +	hp = tty->driver_data;
>> +	if (!hp) {
>> +		rc = -EIO;
>> +		goto out;
>> +	}
>> +
>>  	spin_lock_irqsave(&hp->port.lock, flags);
>>  	/* Check and then increment for fast path open. */
>>  	if (hp->port.count++ > 0) {
>>  		spin_unlock_irqrestore(&hp->port.lock, flags);
>>  		hvc_kick();
>> -		return 0;
>> +		goto out;
>>  	} /* else count == 0 */
>>  	spin_unlock_irqrestore(&hp->port.lock, flags);
> 
> Wait, why isn't this driver just calling tty_port_open() instead of
> trying to open-code all of this?
> 
> Keeping a single mutext for open will not protect it from close, it 
> will
> just slow things down a bit.  There should already be a tty lock held 
> by
> the tty core for open() to keep it from racing things, right?
The tty lock should have been held, but not likely across ->install() 
and
->open() callbacks, thus resulting in a race between hvc_install() and 
hvc_open(),
where hvc_install() sets a data and the hvc_open() clears it. hvc_open() 
doesn't
check if the data was set to NULL and proceeds.
> 
> Try just removing all of this logic and replacing it with a call to
> tty_port_open() and see if that fixes this issue.
> 
> As "proof" of this, I don't see other serial drivers needing a single
> mutex for their open calls, do you?
> 
> thanks,
> 
> greg k-h

Thank you.
Raghavendra