[PATCH v3 1/2] fcntl: fix potential deadlocks for &fown_struct.lock

[Desmond Cheong Zhi Xi]

Syzbot reports a potential deadlock in do_fcntl:

========================================================
WARNING: possible irq lock inversion dependency detected
5.12.0-syzkaller #0 Not tainted
--------------------------------------------------------
syz-executor132/8391 just changed the state of lock:
ffff888015967bf8 (&f->f_owner.lock){.+..}-{2:2}, at: f_getown_ex fs/fcntl.c:211 [inline]
ffff888015967bf8 (&f->f_owner.lock){.+..}-{2:2}, at: do_fcntl+0x8b4/0x1200 fs/fcntl.c:395
but this lock was taken by another, HARDIRQ-safe lock in the past:
 (&dev->event_lock){-...}-{2:2}

and interrupts could create inverse lock ordering between them.

other info that might help us debug this:
Chain exists of:
  &dev->event_lock --> &new->fa_lock --> &f->f_owner.lock

 Possible interrupt unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&f->f_owner.lock);
                               local_irq_disable();
                               lock(&dev->event_lock);
                               lock(&new->fa_lock);
  <Interrupt>
    lock(&dev->event_lock);

 *** DEADLOCK ***

This happens because there is a lock hierarchy of
&dev->event_lock --> &new->fa_lock --> &f->f_owner.lock
from the following call chain:

  input_inject_event():
    spin_lock_irqsave(&dev->event_lock,...);
    input_handle_event():
      input_pass_values():
        input_to_handler():
          evdev_events():
            evdev_pass_values():
              spin_lock(&client->buffer_lock);
              __pass_event():
                kill_fasync():
                  kill_fasync_rcu():
                    read_lock(&fa->fa_lock);
                    send_sigio():
                      read_lock_irqsave(&fown->lock,...);

However, since &dev->event_lock is HARDIRQ-safe, interrupts have to be
disabled while grabbing &f->f_owner.lock, otherwise we invert the lock
hierarchy.

Hence, we replace calls to read_lock/read_unlock on &f->f_owner.lock,
with read_lock_irq/read_unlock_irq.

Here read_lock_irq/read_unlock_irq are safe to use because the
functions f_getown_ex and f_getowner_uids are only called from
do_fcntl, and f_getown is only called from do_fnctl and
sock_ioctl. do_fnctl itself is only called from syscalls.

For sock_ioctl, the chain is
  compat_sock_ioctl():
    compat_sock_ioctl_trans():
      sock_ioctl()

Interrupts are not disabled on either path.

Reported-and-tested-by: syzbot+e6d5398a02c516ce5e70 at syzkaller.appspotmail.com
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx at gmail.com>
---
 fs/fcntl.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/fs/fcntl.c b/fs/fcntl.c
index dfc72f15be7f..cf9e81dfa615 100644
--- a/fs/fcntl.c
+++ b/fs/fcntl.c
@@ -150,7 +150,8 @@ void f_delown(struct file *filp)
 pid_t f_getown(struct file *filp)
 {
 	pid_t pid = 0;
-	read_lock(&filp->f_owner.lock);
+
+	read_lock_irq(&filp->f_owner.lock);
 	rcu_read_lock();
 	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type)) {
 		pid = pid_vnr(filp->f_owner.pid);
@@ -158,7 +159,7 @@ pid_t f_getown(struct file *filp)
 			pid = -pid;
 	}
 	rcu_read_unlock();
-	read_unlock(&filp->f_owner.lock);
+	read_unlock_irq(&filp->f_owner.lock);
 	return pid;
 }
 
@@ -208,7 +209,7 @@ static int f_getown_ex(struct file *filp, unsigned long arg)
 	struct f_owner_ex owner = {};
 	int ret = 0;
 
-	read_lock(&filp->f_owner.lock);
+	read_lock_irq(&filp->f_owner.lock);
 	rcu_read_lock();
 	if (pid_task(filp->f_owner.pid, filp->f_owner.pid_type))
 		owner.pid = pid_vnr(filp->f_owner.pid);
@@ -231,7 +232,7 @@ static int f_getown_ex(struct file *filp, unsigned long arg)
 		ret = -EINVAL;
 		break;
 	}
-	read_unlock(&filp->f_owner.lock);
+	read_unlock_irq(&filp->f_owner.lock);
 
 	if (!ret) {
 		ret = copy_to_user(owner_p, &owner, sizeof(owner));
@@ -249,10 +250,10 @@ static int f_getowner_uids(struct file *filp, unsigned long arg)
 	uid_t src[2];
 	int err;
 
-	read_lock(&filp->f_owner.lock);
+	read_lock_irq(&filp->f_owner.lock);
 	src[0] = from_kuid(user_ns, filp->f_owner.uid);
 	src[1] = from_kuid(user_ns, filp->f_owner.euid);
-	read_unlock(&filp->f_owner.lock);
+	read_unlock_irq(&filp->f_owner.lock);
 
 	err  = put_user(src[0], &dst[0]);
 	err |= put_user(src[1], &dst[1]);
-- 
2.25.1