[PATCH nf v2] netfilter: ip6t_rpfilter: reject routes without inet6_dev

ip6_route_lookup() can return an error-free route whose rt6i_idev is
NULL. 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. An unprivileged user can construct this state with rtnetlink
in a private user and network namespace, then trigger a NULL dereference
through an IPv6 rpfilter lookup:

  Oops: general protection fault, probably for non-canonical address
  0xdffffc0000000000
  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: rpfilter_mt (net/ipv6/netfilter/ip6t_rpfilter.c:75)
  Call Trace:
  ip6t_do_table (net/ipv6/netfilter/ip6_tables.c:316)
  nf_hook_slow (net/netfilter/core.c:619)
  ipv6_rcv (net/ipv6/ip6_input.c:351)
  __netif_receive_skb_one_core (net/core/dev.c:6216)
  process_backlog (net/core/dev.c:6680)
  __napi_poll (net/core/dev.c:7739)
  net_rx_action (net/core/dev.c:7959)
  handle_softirqs (kernel/softirq.c:622)
  do_softirq.part.0 (kernel/softirq.c:523)
  __local_bh_enable_ip (kernel/softirq.c:450)
  __dev_queue_xmit (net/core/dev.c:4913)
  packet_sendmsg (net/packet/af_packet.c:3139)
  __sys_sendto (net/socket.c:2252)
  __x64_sys_sendto (net/socket.c:2259)
  do_syscall_64 (arch/x86/entry/syscall_64.c:94)
  entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
  Kernel panic - not syncing: Fatal exception in interrupt

Reject routes without an inet6_dev immediately after lookup. Such routes
are not eligible for reverse-path filtering, and the check protects all
later rt6i_idev dereferences.

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)
Suggested-by: Florian Westphal <fw@strlen.de>
Assisted-by: Claude:gpt-5
Cc: stable@vger.kernel.org
Signed-off-by: Weiming Shi <redacted>
---
v2:
- Reject routes without inet6_dev in both loose and strict modes, as
  suggested by Florian.
v1: https://lore.kernel.org/all/20260906034939.3746540-1-bestswngs@gmail.com/ (local)
---
 net/ipv6/netfilter/ip6t_rpfilter.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/netfilter/ip6t_rpfilter.c b/net/ipv6/netfilter/ip6t_rpfilter.c
index 67c87a88cde4f..b5def30c3127e 100644
--- a/net/ipv6/netfilter/ip6t_rpfilter.c
+++ b/net/ipv6/netfilter/ip6t_rpfilter.c

@@ -61,7 +61,7 @@ static bool rpfilter_lookup_reverse6(struct net *net, const struct sk_buff *skb,
 		fl6.flowi6_oif = dev->ifindex;
 
 	rt = (void *)ip6_route_lookup(net, &fl6, skb, lookup_flags);
-	if (rt->dst.error)
+	if (rt->dst.error || !rt->rt6i_idev)
 		goto out;
 
 	if (rt->rt6i_flags & (RTF_REJECT|RTF_ANYCAST))

-- 
2.55.0