Re: CVE-2022-1462: race condition vulnerability in drivers/tty/tty_buffers.c

[Jiri Slaby <jirislaby@kernel.org>]

On 02. 06. 22, 6:48, Jiri Slaby wrote:
> On 02. 06. 22, 4:48, Hillf Danton wrote:
>> On Wed, 1 Jun 2022 21:34:26 +0300 Dan Carpenter wrote:
>>> Hi Greg, Jiri,
>>>
>>> I searched lore.kernel.org and it seemed like CVE-2022-1462 might not
>>> have ever been reported to you?  Here is the original email with the
>>> syzkaller reproducer.
>>>
>>> https://seclists.org/oss-sec/2022/q2/155
>>>
>>> The reporter proposed a fix, but it won't work.  Smatch says that some
>>> of the callers are already holding the port->lock.  For example,
>>> sci_dma_rx_complete() will deadlock.
>>
>> Hi Dan
>>
>> To erase the deadlock above, we need to add another helper folding
>> tty_insert_flip_string() and tty_flip_buffer_push() into one nutshell,
>> with buf->tail covered by port->lock.
>>
>> The diff attached in effect reverts
>> 71a174b39f10 ("pty: do tty_flip_buffer_push without port->lock in 
>> pty_write").
>>
>> Only for thoughts now.
> 
> I think this the likely the best approach. Except few points inlined below.
> 
> Another would be to split tty_flip_buffer_push() into two and call only 
> the first one (doing smp_store_release()) inside the lock. I tried that 
> already, but it looks much worse.
> 
> Another would be to add flags to tty_flip_buffer_push(). Like 
> ONLY_ADVANCE and ONLY_QUEUE. Call with the first under the lock, the 
> second outside.
> 
> Ideas, comments?

Apparently not, so Hillf, could you resend your patch after fixing the 
comments below?

Thanks.

>> Hillf
>>
>> +++ b/drivers/tty/pty.c
>> @@ -116,15 +116,8 @@ static int pty_write(struct tty_struct *
>>       if (tty->flow.stopped)
>>           return 0;
>> -    if (c > 0) {
>> -        spin_lock_irqsave(&to->port->lock, flags);
>> -        /* Stuff the data into the input queue of the other end */
>> -        c = tty_insert_flip_string(to->port, buf, c);
>> -        spin_unlock_irqrestore(&to->port->lock, flags);
>> -        /* And shovel */
>> -        if (c)
>> -            tty_flip_buffer_push(to->port);
>> -    }
>> +    if (c > 0)
>> +        c = tty_flip_insert_and_push_buffer(to->port, buf, c);
>>       return c;
>>   }
>> +++ b/drivers/tty/tty_buffer.c
>> @@ -554,6 +554,26 @@ void tty_flip_buffer_push(struct tty_por
>>   }
>>   EXPORT_SYMBOL(tty_flip_buffer_push);
>> +int tty_flip_insert_and_push_buffer(struct tty_port *port, const 
>> unsigned char *string, int cnt)
> 
> It should be _insert_string_, IMO.
> 
>> +{
>> +    struct tty_bufhead *buf = &port->buf;
>> +    unsigned long flags;
>> +
>> +    spin_lock_irqsave(&port->lock, flags);
>> +    cnt = tty_insert_flip_string(port, string, cnt);
>> +    if (cnt) {
>> +        /*
>> +         * Paired w/ acquire in flush_to_ldisc(); ensures 
>> flush_to_ldisc() sees
>> +         * buffer data.
>> +         */
>> +        smp_store_release(&buf->tail->commit, buf->tail->used);
>> +    }
>> +    spin_unlock_irqrestore(&port->lock, flags);
>> +    queue_work(system_unbound_wq, &buf->work);
> 
> \n here please.
> 
>> +    return cnt;
>> +}
>> +EXPORT_SYMBOL(tty_flip_insert_and_push_buffer);
> 
> No need to export this, right?
> 
> thanks,


-- 
js
suse labs