[PATCH net v3 1/1] ip: orphan prefetched skbs before multicast forwarding

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. 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.

When a 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: [email protected]
Reported-by: Vega <[email protected]>
Signed-off-by: Zhiling Zou <[email protected]>
---
changes in v3:
- Describe UDP early demux consistently for both IPv4 and IPv6.
- Drop the IPv6-specific unicast-MAC discussion from the commit message.
- Leave the BPF-only TC redirect and bridge paths for a separate patchset.
- v2 Link: https://lore.kernel.org/all/f968f497251dedbd1263c322a7c4e3e1a727161d.1786023177.git.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