[PATCH net v2 1/1] ip: orphan prefetched skbs before multicast forwarding

[PATCH net v2 1/1] ip: orphan prefetched skbs before multicast forwarding

[Zhiling Zou]

IPv4 and IPv6 input preserve an skb->sk association installed by
bpf_sk_assign() so that local delivery can use the selected socket under
RCU. IPv6 can also get such an association from udp_v6_early_demux(),
which runs in ip6_rcv_finish_core(), after ip6_rcv_core().

The reproduced UDPv6 packet has a multicast IP destination but a unicast
destination MAC address. It is therefore classified as PACKET_HOST and
passes the UDP early-demux check.

When the multicast packet is not locally deliverable, IPv6 hands the
original skb to ip6_mr_input(). IPv4's ip_mr_input() similarly keeps the
original skb when local delivery is not needed. Either path can put the
skb on an unresolved multicast route queue or forward it after the
receive-side RCU section ends.

After the prefetched socket is destroyed, a later skb free invokes
sock_pfree() and dereferences the stale skb->sk. Orphan the skb before
each non-local multicast forwarding path. Local delivery retains the
original skb; the existing skb_clone() calls provide multicast forwarding
with a socket-free clone.

Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
Fixes: 08842c43d016 ("udp: no longer touch sk->sk_refcnt in early demux")
Cc: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
---
changes in v2:
- Cover the IPv4 multicast forwarding path.
- Correct the IPv6 early-demux ordering and document the unicast-MAC
  PACKET_HOST condition used by the reproducer.
- Add the BPF socket-assignment and UDP early-demux Fixes tags.
- v1 Link: https://lore.kernel.org/all/02db4590d0161e31a789dcdfa8d1be1a3212ec2e.1785724784.git.zhilinz@nebusec.ai/

 net/ipv4/ipmr.c      | 3 +++
 net/ipv6/ip6_input.c | 1 +
 2 files changed, 4 insertions(+)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 1d9a4ac14fcef..e5f2b1c6150d2 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -2213,6 +2213,9 @@ int ip_mr_input(struct sk_buff *skb)
 	if (IPCB(skb)->flags & IPSKB_FORWARDED)
 		goto dont_forward;
 
+	if (!local)
+		skb_orphan(skb);
+
 	mrt = ipmr_rt_fib_lookup(net, skb);
 	if (IS_ERR(mrt)) {
 		kfree_skb(skb);
diff --git a/net/ipv6/ip6_input.c b/net/ipv6/ip6_input.c
index 8972863c93ee5..d332ec60f9150 100644
--- a/net/ipv6/ip6_input.c
+++ b/net/ipv6/ip6_input.c
@@ -622,6 +622,7 @@ int ip6_mc_input(struct sk_buff *skb)
 		if (deliver) {
 			skb2 = skb_clone(skb, GFP_ATOMIC);
 		} else {
+			skb_orphan(skb);
 			skb2 = skb;
 			skb = NULL;
 		}
-- 
2.43.0

Re: [PATCH net v2 1/1] ip: orphan prefetched skbs before multicast forwarding

[Ido Schimmel]

On Thu, Aug 06, 2026 at 11:18:56PM +0800, Zhiling Zou wrote:
> IPv4 and IPv6 input preserve an skb->sk association installed by
> bpf_sk_assign() so that local delivery can use the selected socket under
> RCU. IPv6 can also get such an association from udp_v6_early_demux(),
> which runs in ip6_rcv_finish_core(), after ip6_rcv_core().

Last sentence is not accurate since it doesn't cover IPv4 which is also
fixed here. I suggest something like:

"Both address families can also prefetch a socket in UDP early demux. In
both paths (BPF and UDP early demux) a reference is not guaranteed to be
held on the socket."

> 
> The reproduced UDPv6 packet has a multicast IP destination but a unicast
> destination MAC address. It is therefore classified as PACKET_HOST and
> passes the UDP early-demux check.

This is again specific to IPv6 although IPv4 suffers from the same
problem. I think we can drop this paragraph.

> 
> When the multicast packet is not locally deliverable, IPv6 hands the

s/When the/When a/

> original skb to ip6_mr_input(). IPv4's ip_mr_input() similarly keeps the
> original skb when local delivery is not needed. Either path can put the
> skb on an unresolved multicast route queue or forward it after the
> receive-side RCU section ends.
> 
> After the prefetched socket is destroyed, a later skb free invokes
> sock_pfree() and dereferences the stale skb->sk. Orphan the skb before
> each non-local multicast forwarding path. Local delivery retains the
> original skb; the existing skb_clone() calls provide multicast forwarding
> with a socket-free clone.
> 
> Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
> Fixes: 08842c43d016 ("udp: no longer touch sk->sk_refcnt in early demux")
> Cc: stable@vger.kernel.org
> Reported-by: Vega <vega@nebusec.ai>
> Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>

The rest looks fine to me.

Both sashikos mention other possible instances of the bug, but I believe
they require bpf_sk_assign(), unlike UDP early demux which is used here.
Assuming these issues are real and reproducible, they should be fixed in
a separate patchset.