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
