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

On 8/13/2021 3:33 PM, Greg KH wrote:

quoted

Signed-off-by: Nguyen Dinh Phi <redacted>
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.
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