| Date | Tue, 30 Mar 2010 15:48:23 -0700 | From | Greg KH <> | Subject | [03/45] Fix race in tty_fasync() properly |
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2.6.27-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 @@ -2437,8 +2437,10 @@ 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 @@ -200,9 +200,7 @@ static int setfl(int fd, struct file * f 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, 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,
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