GIT: unionfs2-2.6.27.y: Fix race in tty_fasync() properly

[Erez Zadok]

commit 24fce8f6a79db04afef0c6118f6ecdcfec12ffc4
Author: Linus Torvalds <torvalds at linux-foundation.org>
Date:   Sun Feb 7 10:11:23 2010 -0800

    Fix race in tty_fasync() properly
    
    commit 80e1e823989ec44d8e35bdfddadbddcffec90424 upstream.
    
    This reverts commit 703625118069 ("tty: fix race in tty_fasync") and
    commit b04da8bfdfbb ("fnctl: f_modown should call write_lock_irqsave/
    restore") that tried to fix up some of the fallout but was incomplete.
    
    It turns out that we really cannot hold 'tty->ctrl_lock' over calling
    __f_setown, because not only did that cause problems with interrupt
    disables (which the second commit fixed), it also causes a potential
    ABBA deadlock due to lock ordering.
    
    Thanks to Tetsuo Handa for following up on the issue, and running
    lockdep to show the problem.  It goes roughly like this:
    
     - f_getown gets filp->f_owner.lock for reading without interrupts
       disabled, so an interrupt that happens while that lock is held can
       cause a lockdep chain from f_owner.lock -> sighand->siglock.
    
     - at the same time, the tty->ctrl_lock -> f_owner.lock chain that
       commit 703625118069 introduced, together with the pre-existing
       sighand->siglock -> tty->ctrl_lock chain means that we have a lock
       dependency the other way too.
    
    So instead of extending tty->ctrl_lock over the whole __f_setown() call,
    we now just take a reference to the 'pid' structure while holding the
    lock, and then release it after having done the __f_setown.  That still
    guarantees that 'struct pid' won't go away from under us, which is all
    we really ever needed.
    
    Reported-and-tested-by: Tetsuo Handa <penguin-kernel at I-love.SAKURA.ne.jp>
    Acked-by: Greg Kroah-Hartman <gregkh at suse.de>
    Acked-by: Américo Wang <xiyou.wangcong at gmail.com>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh at suse.de>

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index c4b82c7..e6788f4 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -2437,8 +2437,10 @@ static int tty_fasync(int fd, struct file *filp, int on)
 			pid = task_pid(current);
 			type = PIDTYPE_PID;
 		}
-		retval = __f_setown(filp, pid, type, 0);
+		get_pid(pid);
 		spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+		retval = __f_setown(filp, pid, type, 0);
+		put_pid(pid);
 		if (retval)
 			goto out;
 	} else {
diff --git a/fs/fcntl.c b/fs/fcntl.c
index 4eed4d6..ac79b7e 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -200,9 +200,7 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
 static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
                      uid_t uid, uid_t euid, int force)
 {
-	unsigned long flags;
-
-	write_lock_irqsave(&filp->f_owner.lock, flags);
+	write_lock_irq(&filp->f_owner.lock);
 	if (force || !filp->f_owner.pid) {
 		put_pid(filp->f_owner.pid);
 		filp->f_owner.pid = get_pid(pid);
@@ -210,7 +208,7 @@ static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
 		filp->f_owner.uid = uid;
 		filp->f_owner.euid = euid;
 	}
-	write_unlock_irqrestore(&filp->f_owner.lock, flags);
+	write_unlock_irq(&filp->f_owner.lock);
 }
 
 int __f_setown(struct file *filp, struct pid *pid, enum pid_type type,