TTY: open/hangup race fixup

Like in the "TTY: don't allow reopen when ldisc is changing" patch,
this one fixes a TTY WARNING as described in the option 1) there:
1) __tty_hangup from tty_ldisc_hangup to tty_ldisc_enable. During this
section tty_lock is held. However tty_lock is temporarily dropped in
the middle of the function by tty_ldisc_hangup.

The fix is to introduce a new flag which we set during the unlocked
window and check it in tty_reopen too. The flag is TTY_HUPPING and is
cleared after TTY_HUPPED is set.

While at it, remove duplicate TTY_HUPPED set_bit. The one after
calling ops->hangup seems to be more correct. But anyway, we hold
tty_lock, so there should be no difference.

Also document the function it does that kind of crap.

Nicely reproducible with two forked children:
static void do_work(const char *tty)
{
	if (signal(SIGHUP, SIG_IGN) == SIG_ERR) exit(1);
	setsid();
	while (1) {
		int fd = open(tty, O_RDWR|O_NOCTTY);
		if (fd < 0) continue;
		if (ioctl(fd, TIOCSCTTY)) continue;
		if (vhangup()) continue;
		close(fd);
	}
	exit(0);
}

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Reported-by: <Valdis.Kletnieks@vt.edu>
Reported-by: Kyle McMartin <kyle@mcmartin.ca>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

1 files changed, 9 insertions, 1 deletions

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 878f6d667b17..35480dd57a30 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -559,6 +559,9 @@ void __tty_hangup(struct tty_struct *tty)
 
         tty_lock();
 
+        /* some functions below drop BTM, so we need this bit */
+        set_bit(TTY_HUPPING, &tty->flags);
+
         /* inuse_filps is protected by the single tty lock,
            this really needs to change if we want to flush the
            workqueue with the lock held */
@@ -578,6 +581,10 @@ void __tty_hangup(struct tty_struct *tty)
         }
         spin_unlock(&tty_files_lock);
 
+        /*
+         * it drops BTM and thus races with reopen
+         * we protect the race by TTY_HUPPING
+         */
         tty_ldisc_hangup(tty);
 
         read_lock(&tasklist_lock);
@@ -615,7 +622,6 @@ void __tty_hangup(struct tty_struct *tty)
         tty->session = NULL;
         tty->pgrp = NULL;
         tty->ctrl_status = 0;
-        set_bit(TTY_HUPPED, &tty->flags);
         spin_unlock_irqrestore(&tty->ctrl_lock, flags);
 
         /* Account for the p->signal references we killed */
@@ -641,6 +647,7 @@ void __tty_hangup(struct tty_struct *tty)
          * can't yet guarantee all that.
          */
         set_bit(TTY_HUPPED, &tty->flags);
+        clear_bit(TTY_HUPPING, &tty->flags);
         tty_ldisc_enable(tty);
 
         tty_unlock();
@@ -1311,6 +1318,7 @@ static int tty_reopen(struct tty_struct *tty)
         struct tty_driver *driver = tty->driver;
 
         if (test_bit(TTY_CLOSING, &tty->flags) ||
+                        test_bit(TTY_HUPPING, &tty->flags) ||
                         test_bit(TTY_LDISC_CHANGING, &tty->flags))
                 return -EIO;