Subject: Re: POSSIBLE BUG: selftests/net/fcnal-test.sh: [FAIL] in vrf "bind - ns-B IPv6 LLA" test

From: Guillaume Nault <[email-masked]>
Date: Tue, 6 Jun 2023 20:50:52 +0200

On Tue, Jun 06, 2023 at 04:28:02PM +0200, Mirsad Todorovac wrote:
> On 6/6/23 16:11, Guillaume Nault wrote:
> > On Tue, Jun 06, 2023 at 03:57:35PM +0200, Mirsad Todorovac wrote:
> > > +       if (oif) {
> > > +               rcu_read_lock();
> > > +               dev = dev_get_by_index_rcu(net, oif);
> > > +               rcu_read_unlock();
> > 
> > You can't assume '*dev' is still valid after rcu_read_unlock() unless
> > you hold a reference on it.
> > 
> > > +               rtnl_lock();
> > > +               mdev = netdev_master_upper_dev_get(dev);
> > > +               rtnl_unlock();
> > 
> > Because of that, 'dev' might have already disappeared at the time
> > netdev_master_upper_dev_get() is called. So it may dereference an
> > invalid pointer here.
> 
> Good point, thanks. I didn't expect those to change.
> 
> This can be fixed, provided that RCU and RTNL locks can be nested:

Well, yes and no. You can call rcu_read_{lock,unlock}() while under the
rtnl protection, but not the other way around.

>         rcu_read_lock();
>         if (oif) {
>                 dev = dev_get_by_index_rcu(net, oif);
>                 rtnl_lock();
>                 mdev = netdev_master_upper_dev_get(dev);
>                 rtnl_unlock();
>         }

This is invalid: rtnl_lock() uses a mutex, so it can sleep and that's
forbidden inside an RCU critical section.

>         if (sk->sk_bound_dev_if) {
>                 bdev = dev_get_by_index_rcu(net, sk->sk_bound_dev_if);
>         }
> 
>         addr_type = ipv6_addr_type(daddr);
>         if ((__ipv6_addr_needs_scope_id(addr_type) && !oif) ||
>             (addr_type & IPV6_ADDR_MAPPED) ||
>             (oif && sk->sk_bound_dev_if && oif != sk->sk_bound_dev_if &&
>                     !(mdev && sk->sk_bound_dev_if && bdev && mdev == bdev))) {
>                 rcu_read_unlock();
>                 return -EINVAL;
> 	}
>         rcu_read_unlock();
> 
> But again this is still probably not race-free (bdev might also disappear before
> the mdev == bdev test), even if it passed fcnal-test.sh, there is much duplication
> of code, so your one-line solution is obviously by far better. :-)

The real problem is choosing the right function for getting the master
device. In particular netdev_master_upper_dev_get() was a bad choice.
It forces you to take the rtnl, which is unnatural here and obliges you
to add extra code, while all this shouldn't be necessary in the first
place.

> Much obliged.
> Best regards,
> Mirsad