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Subject[05/93] Fix race in tty_fasync() properly
2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Linus Torvalds <torvalds@linux-foundation.org>

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@I-love.SAKURA.ne.jp>
Acked-by: Greg Kroah-Hartman <gregkh@suse.de>
Acked-by: Américo Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
drivers/char/tty_io.c | 4 +++-
fs/fcntl.c | 6 ++----
2 files changed, 5 insertions(+), 5 deletions(-)
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1930,8 +1930,10 @@ static int tty_fasync(int fd, struct fil
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 {
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -199,9 +199,7 @@ static int setfl(int fd, struct file * f
static void f_modown(struct file *filp, struct pid *pid, enum pid_type type,
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);
@@ -213,7 +211,7 @@ static void f_modown(struct file *filp,
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,

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