[PATCH 5.15.y 3/3] ip: orphan prefetched skbs before multicast forwarding

[Sasha Levin <sashal@kernel.org>]

From: Zhiling Zou <zhilinz@nebusec.ai>

[ Upstream commit e36ce6e78fe3fc3c071a26750783b7ba081ce10d ]

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: stable@vger.kernel.org
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reported-by: Vega <vega@nebusec.ai>
Signed-off-by: Zhiling Zou <zhilinz@nebusec.ai>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/0c52eb3d7532aaf8bccf37e0f7c922143c639735.1786552223.git.zhilinz@nebusec.ai
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 5e582eab1ea19..5ef20e8467e31 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c
@@ -2100,6 +2100,9 @@ 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 94d2ab8689d88..403697fa5569d 100644
--- a/net/ipv6/ip6_input.c
+++ b/net/ipv6/ip6_input.c
@@ -555,6 +555,7 @@ int ip6_mc_input(struct sk_buff *skb)
 		if (deliver) {
 			skb2 = skb_clone(skb, GFP_ATOMIC);
 		} else {
+			skb_orphan(skb);
 			skb2 = skb;
 			skb = NULL;
 		}
-- 
2.53.0