Re: [PATCH net] ipv6: rpl: add NULL check for idev in ipv6_rpl_srh_rcv()

[Eric Dumazet]

On Tue, Apr 28, 2026 at 3:48 PM Andrea Mayer <andrea.mayer@xxxxxxxxxxx> wrote:
>
> ipv6_rpl_srh_rcv() dereferences idev from __in6_dev_get() without
> a NULL check when reading idev->cnf.rpl_seg_enabled.
> When the device's MTU drops below IPV6_MIN_MTU, addrconf_ifdown()
> clears dev->ip6_ptr through RCU_INIT_POINTER(), which is immediately
> visible to concurrent readers. A packet that already passed the idev
> check in ip6_rcv_core() can race with this and hit a NULL pointer
> dereference.
>
> Reproduced by flooding traffic through a route with RPL source routing
> while rapidly flapping the receiving interface's MTU between 1500 and
> 1200:
>
>  BUG: KASAN: null-ptr-deref in ipv6_rpl_srh_rcv+0xae/0x1050
>  Read of size 4 at addr 00000000000006b4 by task ping6/318
>
>  CPU: 0 UID: 0 PID: 318 Comm: ping6 Not tainted 7.1.0-rc1-micro-vm-dev-g46f74a3f7d57 #82 PREEMPT(full)
>  Call Trace:
>   <IRQ>
>   kasan_report+0xc6/0x100
>   ipv6_rpl_srh_rcv+0xae/0x1050
>   ip6_protocol_deliver_rcu+0x717/0x960
>   ip6_input_finish+0xa3/0x1b0
>   ip6_input+0xdc/0x490
>   ipv6_rcv+0x338/0x460
>   __netif_receive_skb_one_core+0xd1/0x130
>   process_backlog+0x2c7/0x9f0
>   __napi_poll.constprop.0+0x51/0x270
>   net_rx_action+0x322/0x730
>   handle_softirqs+0x119/0x640
>   do_softirq+0xae/0xe0
>   </IRQ>
>
> Add a NULL check for idev after __in6_dev_get() and drop the skb if
> idev is NULL, consistent with the SRv6 fix in commit 064137935262
> ("ipv6: add NULL checks for idev in SRv6 paths").
>
> Fixes: 8610c7c6e3bd ("net: ipv6: add support for rpl sr exthdr")
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Andrea Mayer <andrea.mayer@xxxxxxxxxxx>
> ---
>  net/ipv6/exthdrs.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/net/ipv6/exthdrs.c b/net/ipv6/exthdrs.c
> index 03cbce842c1a..e398a8851031 100644
> --- a/net/ipv6/exthdrs.c
> +++ b/net/ipv6/exthdrs.c
> @@ -499,6 +499,10 @@ static int ipv6_rpl_srh_rcv(struct sk_buff *skb)
>         u32 r;
>
>         idev = __in6_dev_get(skb->dev);
> +       if (!idev) {
> +               kfree_skb(skb);

I suggest:

kfree_skb_reason(skb, SKB_DROP_REASON_IPV6DISABLED)