Commit 80e1e823 authored Feb 07, 2010 by Linus Torvalds

Fix race in tty_fasync() properly



This reverts commit 70362511 ("tty: fix race in tty_fasync") and
commit b04da8bf ("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 70362511 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@I-love.SAKURA.ne.jp>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-by: Américo Wang <xiyou.wangcong@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 29275254

drivers/char/tty_io.c

+3 −1

Original line number	Original line	Diff line number	Diff line
	@@ -1951,8 +1951,10 @@ static int tty_fasync(int fd, struct file *filp, int on)
	pid = task_pid(current);		pid = task_pid(current);
	type = PIDTYPE_PID;		type = PIDTYPE_PID;
	}		}
	retval = __f_setown(filp, pid, type, 0);		get_pid(pid);
	spin_unlock_irqrestore(&tty->ctrl_lock, flags);		spin_unlock_irqrestore(&tty->ctrl_lock, flags);
			retval = __f_setown(filp, pid, type, 0);
			put_pid(pid);
	if (retval)		if (retval)
	goto out;		goto out;
	} else {		} else {

fs/fcntl.c

+2 −4

Original line number	Original line	Diff line number	Diff line
	@@ -199,9 +199,7 @@ 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,		static void f_modown(struct file filp, struct pid pid, enum pid_type type,
	int force)		int force)
	{		{
	unsigned long flags;		write_lock_irq(&filp->f_owner.lock);

	write_lock_irqsave(&filp->f_owner.lock, flags);
	if (force \|\| !filp->f_owner.pid) {		if (force \|\| !filp->f_owner.pid) {
	put_pid(filp->f_owner.pid);		put_pid(filp->f_owner.pid);
	filp->f_owner.pid = get_pid(pid);		filp->f_owner.pid = get_pid(pid);
	@@ -213,7 +211,7 @@ static void f_modown(struct file filp, struct pid pid, enum pid_type type,
	filp->f_owner.euid = cred->euid;		filp->f_owner.euid = cred->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,		int __f_setown(struct file filp, struct pid pid, enum pid_type type,