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

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 6/17/26 2:55 PM, Weiming Shi wrote:
> 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>


Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>