From 80e1e823989ec44d8e35bdfddadbddcffec90424 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 7 Feb 2010 10:11:23 -0800 Subject: [PATCH] Fix race in tty_fasync() properly MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Acked-by: Greg Kroah-Hartman Acked-by: Américo Wang Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- drivers/char/tty_io.c | 4 +++- fs/fcntl.c | 6 ++---- 2 files changed, 5 insertions(+), 5 deletions(-) diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c index c6f3b48be9dd..dcb9083ecde0 100644 --- a/drivers/char/tty_io.c +++ b/drivers/char/tty_io.c @@ -1951,8 +1951,10 @@ 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 5ef953e6f908..97e01dc0d95f 100644 --- a/fs/fcntl.c +++ b/fs/fcntl.c @@ -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, 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, struct pid *pid, enum pid_type type, 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, -- 2.20.1