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

[Jiri Slaby <jirislaby@kernel.org>]

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?

> 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