linux-kernel - [PATCH] drivers:tty:pty: Fix a race causing data loss on close

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Date:   Thu,  1 Oct 2020 21:16:30 -0500
From:   minyard@....org
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jirislaby@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Corey Minyard <cminyard@...sta.com>
Subject: [PATCH] drivers:tty:pty: Fix a race causing data loss on close

From: Corey Minyard <cminyard@...sta.com>

If you write to a pty master an immediately close the pty master, the
receiver might get a chunk of data dropped, but then receive some later
data.  That's obviously something rather unexpected for a user.  It
certainly confused my test program.

It turns out that tty_vhangup() gets called from pty_close(), and that
causes the data on the slave side to be flushed, but due to races more
data can be copied into the slave side's buffer after that.  Consider
the following sequence:

thread1            thread2            thread3
                   write data into buffer,
                      n_tty buffer is filled
                   pty_close()
                    tty_vhangup()
                     tty_ldisc_hangup()
                      n_tty_flush_buffer()
                       reset_buffer_flags()
n_tty_read()
 up_read(&tty->termios_rwsem);
                        down_read(&tty->termios_rwsem);
                        clear n_tty buffer contents
                        up_read(&tty->termios_rwsem);
 tty_buffer_flush_work()
  schedules work calling
        flush_to_ldisc()
                                      flush_to_ldisc()
                                       receive_buf()
                                        tty_port_default_receive_buf()
                                         tty_ldisc_receive_buf()
                                          tty_ldisc_receive_buf()
                                           n_tty_receive_buf2()
                                            n_tty_receive_buf_common()
                                             down_read(&tty->termios_rwsem);
                                             __receive_buf()
                                              copies data into n_tty buffer
                                             up_read(&tty->termios_rwsem);
 down_read(&tty->termios_rwsem);
 copy buffer data to user

This change checks to see if the tty is being hung up before copying
anything in n_tty_receive_buf_common().  It has to be done after the
tty->termios_rwsem semaphore is claimed, for reasons that should be
apparent from the sequence above.

Signed-off-by: Corey Minyard <cminyard@...sta.com>
---
I sent a program to reproduce this, I extended it to prove it wasn't the
test program that caused the issue, and I've uploaded it to:
  http://sourceforge.net/projects/ser2net/files/tmp/testpty.c
if you want to run it.  It has a lot of comments that explain what is
going on.

This is not a very satisfying fix, though.  It works reliably, but it
doesn't seem right to me.  My inclination was to remove the up and down
semaphore around tty_buffer_flush_work() in n_tty_read(), as it just
schedules some work, no need to unlock for that.  But that resulted
in a deadlock elsewhere, so that up/down on the semaphore is there for
another reason.

The locking in the tty code is really hard to follow.  I believe this is
actually a locking problem, but fixing it looks daunting to me.

-corey

 drivers/tty/n_tty.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 1794d84e7bf6..1c33c26dc229 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -1704,6 +1704,9 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,

 	down_read(&tty->termios_rwsem);

+	if (test_bit(TTY_HUPPING, &tty->flags))
+		goto out_upsem;
+
 	do {
 		/*
 		 * When PARMRK is set, each input char may take up to 3 chars
@@ -1760,6 +1763,7 @@ n_tty_receive_buf_common(struct tty_struct *tty, const unsigned char *cp,
 	} else
 		n_tty_check_throttle(tty);

+out_upsem:
 	up_read(&tty->termios_rwsem);

 	return rcvd;
-- 
2.17.1