[PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Nguyen Dinh Phi]

The ops->receive_buf() may be accessed concurrently from these two
functions.  If the driver flushes data to the line discipline
receive_buf() method while tiocsti() is using the ops->receive_buf(),
the data race will happen.

For example:
tty_ioctl                       |tty_ldisc_receive_buf
 ->tioctsi                      | ->tty_port_default_receive_buf
                                |  ->tty_ldisc_receive_buf
   ->hci_uart_tty_receive       |   ->hci_uart_tty_receive
    ->h4_recv                   |    ->h4_recv

In this case, the h4 receive buffer will be overwritten by the
latecomer, and it cause a memory leak.

Hence, change tioctsi() function to use the exclusive lock interface
from tty_buffer to avoid the data race.

Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
---
 drivers/tty/tty_io.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e8532006e960..746fe13a2054 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
 	ld = tty_ldisc_ref_wait(tty);
 	if (!ld)
 		return -EIO;
+	tty_buffer_lock_exclusive(tty->port);
 	if (ld->ops->receive_buf)
 		ld->ops->receive_buf(tty, &ch, &mbz, 1);
+	tty_buffer_unlock_exclusive(tty->port);
 	tty_ldisc_deref(ld);
 	return 0;
 }
--
2.25.1

Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Greg KH]

On Sun, Aug 08, 2021 at 03:05:13AM +0800, Nguyen Dinh Phi wrote:
> The ops->receive_buf() may be accessed concurrently from these two
> functions.  If the driver flushes data to the line discipline
> receive_buf() method while tiocsti() is using the ops->receive_buf(),
> the data race will happen.
> 
> For example:
> tty_ioctl                       |tty_ldisc_receive_buf
>  ->tioctsi                      | ->tty_port_default_receive_buf
>                                 |  ->tty_ldisc_receive_buf
>    ->hci_uart_tty_receive       |   ->hci_uart_tty_receive
>     ->h4_recv                   |    ->h4_recv
> 
> In this case, the h4 receive buffer will be overwritten by the
> latecomer, and it cause a memory leak.

That looks to be a bug in the h4 code, if the receive_buf() call can not
be run at the same time, it should have a lock in it, right?

> Hence, change tioctsi() function to use the exclusive lock interface
> from tty_buffer to avoid the data race.

Where is the lock being grabbed from the other receive_buf() call path
to ensure that the lock is always needed here?

> 
> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
> Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
> ---
>  drivers/tty/tty_io.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index e8532006e960..746fe13a2054 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>  	ld = tty_ldisc_ref_wait(tty);
>  	if (!ld)
>  		return -EIO;
> +	tty_buffer_lock_exclusive(tty->port);
>  	if (ld->ops->receive_buf)
>  		ld->ops->receive_buf(tty, &ch, &mbz, 1);
> +	tty_buffer_unlock_exclusive(tty->port);

Did this fix the syzbot reported issue?

thanks,

greg k-h

Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Phi Nguyen]

On 8/13/2021 3:33 PM, Greg KH wrote:
>>
>> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
>> Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
>> ---
>>   drivers/tty/tty_io.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>> index e8532006e960..746fe13a2054 100644
>> --- a/drivers/tty/tty_io.c
>> +++ b/drivers/tty/tty_io.c
>> @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>>   	ld = tty_ldisc_ref_wait(tty);
>>   	if (!ld)
>>   		return -EIO;
>> +	tty_buffer_lock_exclusive(tty->port);
>>   	if (ld->ops->receive_buf)
>>   		ld->ops->receive_buf(tty, &ch, &mbz, 1);
>> +	tty_buffer_unlock_exclusive(tty->port);
> 
> Did this fix the syzbot reported issue?
> 
> thanks,
> 
> greg k-h
> Yes, this fixed the syzbot reported issue.

The lock is grabbed in flush_to_ldisc() and paste_selection().
Actually, I follow the document in tty_buffer.c, where it say the 
callers to receive_buff() other than flush_to_ldisc() need to exclude 
the kworker from concurrent use of the line discipline.

And function tiocsti() has the following comment:
/* FIXME: may race normal receive processing */
that why I add lock in this function.

BR,
Phi.

Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Greg KH]

On Sat, Aug 14, 2021 at 02:35:53AM +0800, Phi Nguyen wrote:
> On 8/13/2021 3:33 PM, Greg KH wrote:
> > > 
> > > Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
> > > Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
> > > ---
> > >   drivers/tty/tty_io.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> > > index e8532006e960..746fe13a2054 100644
> > > --- a/drivers/tty/tty_io.c
> > > +++ b/drivers/tty/tty_io.c
> > > @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
> > >   	ld = tty_ldisc_ref_wait(tty);
> > >   	if (!ld)
> > >   		return -EIO;
> > > +	tty_buffer_lock_exclusive(tty->port);
> > >   	if (ld->ops->receive_buf)
> > >   		ld->ops->receive_buf(tty, &ch, &mbz, 1);
> > > +	tty_buffer_unlock_exclusive(tty->port);
> > 
> > Did this fix the syzbot reported issue?
> > 
> > thanks,
> > 
> > greg k-h
> > Yes, this fixed the syzbot reported issue.
> 
> The lock is grabbed in flush_to_ldisc() and paste_selection().
> Actually, I follow the document in tty_buffer.c, where it say the callers to
> receive_buff() other than flush_to_ldisc() need to exclude the kworker from
> concurrent use of the line discipline.
> 
> And function tiocsti() has the following comment:
> /* FIXME: may race normal receive processing */
> that why I add lock in this function.

As you are fixing this FIXME, please remove it in this patch, otherwise
we will not know it is resolved :)

Can you add that to the change and submit a new version?

Also, this should go to stable kernels, right?  Any idea how far back?

thanks,

greg k-h

Re: [PATCH] tty: Fix data race between tiocsti() and flush_to_ldisc()

[Phi Nguyen]

On 8/18/2021 10:03 PM, Greg KH wrote:
> On Sat, Aug 14, 2021 at 02:35:53AM +0800, Phi Nguyen wrote:
>> On 8/13/2021 3:33 PM, Greg KH wrote:
>>>>
>>>> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
>>>> Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.appspotmail.com
>>>> ---
>>>>    drivers/tty/tty_io.c | 2 ++
>>>>    1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
>>>> index e8532006e960..746fe13a2054 100644
>>>> --- a/drivers/tty/tty_io.c
>>>> +++ b/drivers/tty/tty_io.c
>>>> @@ -2307,8 +2307,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>>>>    	ld = tty_ldisc_ref_wait(tty);
>>>>    	if (!ld)
>>>>    		return -EIO;
>>>> +	tty_buffer_lock_exclusive(tty->port);
>>>>    	if (ld->ops->receive_buf)
>>>>    		ld->ops->receive_buf(tty, &ch, &mbz, 1);
>>>> +	tty_buffer_unlock_exclusive(tty->port);
>>>
>>> Did this fix the syzbot reported issue?
>>>
>>> thanks,
>>>
>>> greg k-h
>>> Yes, this fixed the syzbot reported issue.
>>
>> The lock is grabbed in flush_to_ldisc() and paste_selection().
>> Actually, I follow the document in tty_buffer.c, where it say the callers to
>> receive_buff() other than flush_to_ldisc() need to exclude the kworker from
>> concurrent use of the line discipline.
>>
>> And function tiocsti() has the following comment:
>> /* FIXME: may race normal receive processing */
>> that why I add lock in this function.
> 
> As you are fixing this FIXME, please remove it in this patch, otherwise
> we will not know it is resolved :)
> 
> Can you add that to the change and submit a new version?
> 
Yes, I will submit a new version.

> Also, this should go to stable kernels, right?  Any idea how far back?
>
I have no idea about this question, but I see it meets specified 
requirements in stable-kernel-rules.rst

BR,
Phi.