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SubjectRe: [PATCH v2] tty: Fix data race between tiocsti() and flush_to_ldisc()
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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@kernel.org>

> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
> Reported-by: syzbot+97388eb9d31b997fe1d0@syzkaller.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
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