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

[Desmond Cheong Zhi Xi]

On 2/7/21 7:44 pm, Jeff Layton wrote:
> On Fri, 2021-07-02 at 17:18 +0800, Desmond Cheong Zhi Xi wrote:
> > 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.
>
> Patches look reasonable overall, but why does this one use read_lock_irq
> and the other one use read_lock_irqsave? Don't we need to *_irqsasve in
> both patches?

My thinking was that 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()

For both paths, it doesn't seem that interrupts are disabled, so I used 
the *irq variants.

But of course, I might be very mistaken on this, and I'd be happy to 
make the change to *_irqsave.

Also, on further inspection, if these calls should be changed to 
*_irqsave, then I believe the call to write_lock_irq in f_modown (called 
from do_fcntl() --> f_setown() --> __f_setown() --> f_modown()) should 
also be changed to *_irqsave.

There's also a call to write_lock_irq(&fa->fa_lock) in 
fasync_remove_entry and fasync_insert_entry. Whether these should be 
changed as well isn't as clear to me, but since it's safe to do, perhaps 
it makes sense to use *_irqsave for them too. Thoughts?

> > Reported-and-tested-by: syzbot+e6d5398a02c516ce5e70@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > ---
> >   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]);