linux-kernel - Re: [PATCH v2] tty: Fix data race between tiocsti() and flush_to

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Date:   Mon, 23 Aug 2021 10:52:57 +0200
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Nguyen Dinh Phi <phind.uet@...il.com>, gregkh@...uxfoundation.org
Cc:     linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+97388eb9d31b997fe1d0@...kaller.appspotmail.com
Subject: Re: [PATCH v2] tty: Fix data race between tiocsti() and
 flush_to_ldisc()

On 23. 08. 21, 2:06, 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 waiting for the
> ops->receive_buf() to finish its work, 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 we will lost the data.
> 
> Hence, change tioctsi() function to use the exclusive lock interface
> from tty_buffer to avoid the data race.

This makes sense. It mimics paste_selection() -- the lock order is 
preexisting. The normal path processing has the locks inverted, not sure 
why it doesn't matter :):
flush_to_ldisc
   mutex_lock(&buf->lock);    <--------- A (mutex)
   receive_buf()
     tty_port_default_receive_buf()
       disc = tty_ldisc_ref(tty); <----- B (ldsem for READ)

Reviewed-by: Jiri Slaby <jirislaby@...nel.org>

> Signed-off-by: Nguyen Dinh Phi <phind.uet@...il.com>
> Reported-by: syzbot+97388eb9d31b997fe1d0@...kaller.appspotmail.com
> ---
> V2:
> 	- Remove FIXME comment.
> 
>   drivers/tty/tty_io.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index e8532006e960..6616d4a0d41d 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2290,8 +2290,6 @@ static int tty_fasync(int fd, struct file *filp, int on)
>    *	Locking:
>    *		Called functions take tty_ldiscs_lock
>    *		current->signal->tty check is safe without locks
> - *
> - *	FIXME: may race normal receive processing
>    */
> 
>   static int tiocsti(struct tty_struct *tty, char __user *p)
> @@ -2307,8 +2305,10 @@ static int tiocsti(struct tty_struct *tty, char __user *p)
>   	ld = tty_ldisc_ref_wait(tty);

Here (and in paste_selection()), it is lock "B (ldsem) for READ".


>   	if (!ld)
>   		return -EIO;
> +	tty_buffer_lock_exclusive(tty->port);

and "A (mutex)".

>   	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;
>   }

thanks,
-- 
js
suse labs