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>

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 878f6d667b1744aaf90be87a50da56e919413ad4..35480dd57a307e72b2893d484a86783659c6a45d 100644 (file)
--- 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;
 

diff --git a/include/linux/tty.h b/include/linux/tty.h
index c7ea9bc8897cb638208a88bcb6352814b4639dc8..021bfd2f08604aa33950887294dee2f16fe0b9ed 100644 (file)
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -367,6 +367,7 @@ 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))