ipv6: ip6mr: Call ip6mr_fib_lookup() under RCU in pim6_rcv() and reg_vif6_xmit()

[y2k <y2k@desarrollaria.com>]

Commit 019c892e4654 ("ipmr: Call ipmr_fib_lookup() under RCU.") fixed
the same issue in IPv4's reg_vif_xmit(). The IPv6 counterpart has the
same problem in two places.

In pim6_rcv() (net/ipv6/ip6mr.c:578) and reg_vif6_xmit()
(net/ipv6/ip6mr.c:624), ip6mr_fib_lookup() is called without holding
rcu_read_lock().

When CONFIG_IP6_MROUTE_MULTIPLE_TABLES=n, ip6mr_fib_lookup() accesses
net->ipv6.mrt6 directly without rcu_dereference(), while the IPv4
equivalent correctly uses rcu_dereference(net->ipv4.mrt). This
inconsistency means IPv6 multicast routing lacks proper RCU protection.

In reg_vif6_xmit(), rcu_read_lock() is acquired at line 628 after the
ip6mr_fib_lookup() call at line 624 — too late. In pim6_rcv(), there
is no rcu_read_lock() before ip6mr_fib_lookup() at line 578 at all.

Suggested fix for reg_vif6_xmit():

  + rcu_read_lock();
    if (ip6mr_fib_lookup(net, &fl6, &mrt) < 0) {
  +   rcu_read_unlock();
      goto tx_err;
    }
    DEV_STATS_ADD(dev, tx_bytes, skb->len);
    DEV_STATS_INC(dev, tx_packets);
  - rcu_read_lock();
    ip6mr_cache_report(mrt, skb, READ_ONCE(mrt->mroute_reg_vif_num),
                       MRT6MSG_WHOLEPKT);
    rcu_read_unlock();

Suggested fix for pim6_rcv():

  + rcu_read_lock();
    if (ip6mr_fib_lookup(net, &fl6, &mrt) < 0) {
  +   rcu_read_unlock();
      goto drop;
    }

Additionally, net->ipv6.mrt6 should be accessed via rcu_dereference()
in ip6mr_fib_lookup() to match the IPv4 pattern in ipmr_fib_lookup().

Thanks,
y2k
y2k@desarrollaria.com