[PATCH nf] netfilter: ip6t_rpfilter: handle routes without inet6_dev

ip6_route_lookup() can return an error-free route whose rt6i_idev is
NULL.  For example, lowering an external nexthop device's MTU below
IPV6_MIN_MTU tears down its inet6_dev while fib6_ifdown() leaves routes
using nexthop objects in the FIB.

rpfilter_lookup_reverse6() dereferences rt6i_idev before evaluating its
loose-mode condition.  This lets an unprivileged user with a private user
and network namespace trigger a NULL pointer dereference:

  Oops: general protection fault, probably for non-canonical address
  0xdffffc0000000000
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: rpfilter_mt
  ip6t_do_table
  nf_hook_slow
  ipv6_rcv
  process_backlog
  net_rx_action
  handle_softirqs

Evaluate loose mode first because route existence is sufficient there.
In strict mode, compare devices only when the route has an inet6_dev.

Fixes: e26f9a480fb6 ("netfilter: add ipv6 reverse path filter match")
Reported-by: co+459f67f4d8af8ce6@bugs.sh
Closes: https://lore.kernel.org/all/VtWUkE8QzJt5CroTj2V2v3ZQ0gwbXZ7nq7I3@bugs.sh/ (local)
Assisted-by: Claude:gpt-5
Signed-off-by: Weiming Shi <redacted>
---
 net/ipv6/netfilter/ip6t_rpfilter.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/netfilter/ip6t_rpfilter.c b/net/ipv6/netfilter/ip6t_rpfilter.c
index 67c87a88cde4f..2bcc96f9385cb 100644
--- a/net/ipv6/netfilter/ip6t_rpfilter.c
+++ b/net/ipv6/netfilter/ip6t_rpfilter.c

@@ -72,9 +72,11 @@ static bool rpfilter_lookup_reverse6(struct net *net, const struct sk_buff *skb,
 		goto out;
 	}
 
-	if (rt->rt6i_idev->dev == dev ||
-	    l3mdev_master_ifindex_rcu(rt->rt6i_idev->dev) == dev->ifindex ||
-	    (flags & XT_RPFILTER_LOOSE))
+	if (flags & XT_RPFILTER_LOOSE)
+		ret = true;
+	else if (rt->rt6i_idev &&
+		 (rt->rt6i_idev->dev == dev ||
+		  l3mdev_master_ifindex_rcu(rt->rt6i_idev->dev) == dev->ifindex))
 		ret = true;
  out:
 	ip6_rt_put(rt);

-- 
2.55.0