[PATCH] tty: n_tty: read termios under lock in poll

[PATCH] tty: n_tty: read termios under lock in poll

[Cen Zhang]

n_tty_poll() uses input_available_p() to decide whether buffered input
makes the tty readable. That helper reads termios state through
L_EXTPROC(), VMIN, and VTIME, but the poll path does not hold the read
side of tty->termios_rwsem.

tty_set_termios() updates tty->termios under the write side of the same
semaphore, including c_lflag and c_cc[]. n_tty_read() already takes the
read side before reading the same termios fields and before calling
input_available_p(). Protect the poll-side readiness checks the same way
so poll observes a coherent termios state when deciding whether to report
readable input.

Do not hold termios_rwsem across tty_buffer_flush_work(), matching the
read path which drops the semaphore before flushing pending receive work
and then checks input availability again after reacquiring it.

Signed-off-by: Cen Zhang <zzzccc427@gmail.com>
---
 drivers/tty/n_tty.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index e6a0f5b40d0a..c8e1882782db 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -2437,13 +2437,17 @@ static __poll_t n_tty_poll(struct tty_struct *tty, struct file *file,
 
 	poll_wait(file, &tty->read_wait, wait);
 	poll_wait(file, &tty->write_wait, wait);
-	if (input_available_p(tty, 1))
-		mask |= EPOLLIN | EPOLLRDNORM;
-	else {
-		tty_buffer_flush_work(tty->port);
+	scoped_guard(rwsem_read, &tty->termios_rwsem) {
 		if (input_available_p(tty, 1))
 			mask |= EPOLLIN | EPOLLRDNORM;
 	}
+	if (!(mask & (EPOLLIN | EPOLLRDNORM))) {
+		tty_buffer_flush_work(tty->port);
+		scoped_guard(rwsem_read, &tty->termios_rwsem) {
+			if (input_available_p(tty, 1))
+				mask |= EPOLLIN | EPOLLRDNORM;
+		}
+	}
 	if (tty->ctrl.packet && tty->link->ctrl.pktstatus)
 		mask |= EPOLLPRI | EPOLLIN | EPOLLRDNORM;
 	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
-- 
2.43.0

Re: [PATCH] tty: n_tty: read termios under lock in poll

[Greg KH]

On Sun, May 10, 2026 at 10:59:40AM +0800, Cen Zhang wrote:
> n_tty_poll() uses input_available_p() to decide whether buffered input
> makes the tty readable. That helper reads termios state through
> L_EXTPROC(), VMIN, and VTIME, but the poll path does not hold the read
> side of tty->termios_rwsem.
> 
> tty_set_termios() updates tty->termios under the write side of the same
> semaphore, including c_lflag and c_cc[]. n_tty_read() already takes the
> read side before reading the same termios fields and before calling
> input_available_p(). Protect the poll-side readiness checks the same way
> so poll observes a coherent termios state when deciding whether to report
> readable input.

But why does that matter?  If it changes right after you grab/release
the lock, the data will be stale as well.  What userspace logic is
broken because of there not being a lock held here?

thanks,

greg k-h