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:
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)
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
Signed-off-by: Weiming Shi
---
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