[PATCH net] ipv6: ndisc: fix NULL deref in accept_untracked_na()

[Weiming Shi <bestswngs@gmail.com>]

accept_untracked_na() re-fetches the inet6_dev with __in6_dev_get(dev)
and dereferences idev->cnf.accept_untracked_na without a NULL check,
even though its only caller ndisc_recv_na() already fetched and
NULL-checked idev for the same device.

Both reads of dev->ip6_ptr run in the same RCU read-side critical
section, but a concurrent addrconf_ifdown() can clear dev->ip6_ptr
between them: lowering the MTU below IPV6_MIN_MTU calls addrconf_ifdown()
without the synchronize_net() that orders the unregister path, so the
re-fetch returns NULL and oopses:

 BUG: KASAN: null-ptr-deref in ndisc_recv_na (net/ipv6/ndisc.c:974)
 Read of size 4 at addr 0000000000000364
 Call Trace:
  <IRQ>
  ndisc_recv_na (net/ipv6/ndisc.c:974)
  icmpv6_rcv (net/ipv6/icmp.c:1193)
  ip6_protocol_deliver_rcu (net/ipv6/ip6_input.c:479)
  ip6_input_finish (net/ipv6/ip6_input.c:534)
  ip6_input (net/ipv6/ip6_input.c:545)
  ip6_mc_input (net/ipv6/ip6_input.c:635)
  ipv6_rcv (net/ipv6/ip6_input.c:351)
  </IRQ>

It is reachable by an unprivileged user via a network namespace.

Pass the caller's already validated idev instead of re-fetching it; the
idev stays alive for the whole RCU critical section, so it is safe even
after dev->ip6_ptr has been cleared.

Fixes: aaa5f515b16b ("net: ipv6: new accept_untracked_na option to accept na only if in-network")
Assisted-by: Claude:claude-opus-4-8
Reported-by: Xiang Mei <xmei5@asu.edu>
Signed-off-by: Weiming Shi <bestswngs@gmail.com>

---
 net/ipv6/ndisc.c | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/net/ipv6/ndisc.c b/net/ipv6/ndisc.c
index e7ad13c5bd267..f867ec8d3d905 100644
--- a/net/ipv6/ndisc.c
+++ b/net/ipv6/ndisc.c
@@ -967,10 +967,8 @@ static enum skb_drop_reason ndisc_recv_ns(struct sk_buff *skb)
 	return reason;
 }
 
-static int accept_untracked_na(struct net_device *dev, struct in6_addr *saddr)
+static int accept_untracked_na(struct inet6_dev *idev, struct in6_addr *saddr)
 {
-	struct inet6_dev *idev = __in6_dev_get(dev);
-
 	switch (READ_ONCE(idev->cnf.accept_untracked_na)) {
 	case 0: /* Don't accept untracked na (absent in neighbor cache) */
 		return 0;
@@ -980,7 +978,7 @@ static int accept_untracked_na(struct net_device *dev, struct in6_addr *saddr)
 		 * same subnet as an address configured on the interface that
 		 * received the na
 		 */
-		return !!ipv6_chk_prefix(saddr, dev);
+		return !!ipv6_chk_prefix(saddr, idev->dev);
 	default:
 		return 0;
 	}
@@ -1078,7 +1076,7 @@ static enum skb_drop_reason ndisc_recv_na(struct sk_buff *skb)
 	 */
 	new_state = msg->icmph.icmp6_solicited ? NUD_REACHABLE : NUD_STALE;
 	if (!neigh && lladdr && idev && READ_ONCE(idev->cnf.forwarding)) {
-		if (accept_untracked_na(dev, saddr)) {
+		if (accept_untracked_na(idev, saddr)) {
 			neigh = neigh_create(&nd_tbl, &msg->target, dev);
 			new_state = NUD_STALE;
 		}
-- 
2.43.0