TTY: open/hangup race fixup

commit acfa747baf73922021a047f2d87a2d866f5dbab5 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index 615ecf37e28443b9c8642a8f830faac084dd3d09..e0f7f4b8c2869dd3bf60a0a28d1c97e3f628012f 100644 (file)
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -553,6 +553,9 @@ 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 */
@@ -572,6 +575,10 @@ 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);
@@ -609,7 +616,6 @@ 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 */
@@ -635,6 +641,7 @@ 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();
@@ -1305,6 +1312,7 @@ 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;
 

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 67d64e6efe7a8c95d775b6418d2c1c8d728d667d..6f62c305581998ceab9280fd152b00115f29ff16 100644 (file)
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -365,6 +365,7 @@ struct tty_file_private {
 #define TTY_HUPPED             18      /* Post driver->hangup() */
 #define TTY_FLUSHING           19      /* Flushing to ldisc in progress */
 #define TTY_FLUSHPENDING       20      /* Queued buffer flush pending */
+#define TTY_HUPPING            21      /* ->hangup() in progress */
 
 #define TTY_WRITE_FLUSH(tty) tty_write_flush((tty))