netdev - Re: null pointer dereference in interrupt after receiving an ip packet on veth from xsk from user space

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Message-ID: <aPedG99fdFBnbIqz@boxer>
Date: Tue, 21 Oct 2025 16:47:55 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Jason Xing <kerneljasonxing@...il.com>
CC: mc36 <csmate@....hu>, <alekcejk@...glemail.com>, Jonathan Lemon
	<jonathan.lemon@...il.com>, Stanislav Fomichev <sdf@...ichev.me>, "Magnus
 Karlsson" <magnus.karlsson@...el.com>, Björn Töpel
	<bjorn@...nel.org>, <1118437@...s.debian.org>, <netdev@...r.kernel.org>,
	<bpf@...r.kernel.org>
Subject: Re: null pointer dereference in interrupt after receiving an ip
 packet on veth from xsk from user space

On Tue, Oct 21, 2025 at 07:02:06PM +0800, Jason Xing wrote:
> On Tue, Oct 21, 2025 at 5:31 AM mc36 <csmate@....hu> wrote:
> >
> > hi,
> >
> > On 10/20/25 11:04, Jason Xing wrote:
> > >
> > > I followed your steps you attached in your code:
> > > ////// gcc xskInt.c -lxdp
> > > ////// sudo ip link add veth1 type veth
> > > ////// sudo ip link set veth0 up
> > > ////// sudo ip link set veth1 up
> >
> > ip link set dev veth1 address 3a:10:5c:53:b3:5c
> 
> Great, it indeed helps me reproduce the issue, so I managed to see the
> exact same stack. Let me dig into it more deeply.

splat comes from skb_orphan() calling skb->destructor() with ::cb field
being already taken by IP layer. A hotfix would simply be moving this call
before we memset cb in ip_rcv_core():

diff --git a/net/ipv4/ip_input.c b/net/ipv4/ip_input.c
index 273578579a6b..db30645f8c35 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -535,14 +535,14 @@ static struct sk_buff *ip_rcv_core(struct sk_buff *skb, struct net *net)
        iph = ip_hdr(skb);
        skb->transport_header = skb->network_header + iph->ihl*4;

-       /* Remove any debris in the socket control block */
-       memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
-       IPCB(skb)->iif = skb->skb_iif;
-
        /* Must drop socket now because of tproxy. */
        if (!skb_sk_is_prefetched(skb))
                skb_orphan(skb);

+       /* Remove any debris in the socket control block */
+       memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
+       IPCB(skb)->iif = skb->skb_iif;
+
        return skb;

 csum_error:

However, I do not understand why setting mac addr on one veth interface
triggers this path.

> 
> Thanks,
> Jason