ipmr_cache_report() copies ip_hdrlen(pkt) bytes from pkt->data into
a freshly allocated 128-byte skb that is delivered to userspace via
the mrouted IGMP raw socket and via igmpmsg_netlink_event:

  const int ihl = ip_hdrlen(pkt);
  ...
  skb_put(skb, ihl);
  skb_copy_to_linear_data(skb, pkt->data, ihl);

ip_rcv_core() validates iph->ihl and pskb_may_pull()s ihl*4 bytes at
parse time.  An nftables PRE_ROUTING payload write reachable from an
unprivileged user namespace can flip the ihl nibble from 5 to 15
between parse and ipmr_cache_report().  When the original skb is
non-linear (received via a NIC driver that uses paged frags), only
the parse-time ihl*4 = 20 bytes are in the linear region; the
consumer copies 60 bytes, and the extra 40 bytes are read from
skb_shared_info or adjacent slab memory and queued back to userspace,
a kernel heap-content infoleak.  PoC observation: recvfrom on the
mroute socket returns 28 bytes without mutation, 68 bytes with
mutation (40 extra bytes leaked).

Clamp ihl against skb_headlen(pkt) so only bytes actually present
in the linear region are copied.

Cc: stable@vger.kernel.org
Reported-by: Qi Tang <redacted>
Reported-by: Tong Liu <redacted>
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Qi Tang <redacted>
---
 net/ipv4/ipmr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv4/ipmr.c b/net/ipv4/ipmr.c
index 2628cd3a93a68..b40f3dd8f650f 100644
--- a/net/ipv4/ipmr.c
+++ b/net/ipv4/ipmr.c

@@ -1056,7 +1056,7 @@ static void ipmr_cache_resolve(struct net *net, struct mr_table *mrt,
 static int ipmr_cache_report(const struct mr_table *mrt,
 			     struct sk_buff *pkt, vifi_t vifi, int assert)
 {
-	const int ihl = ip_hdrlen(pkt);
+	const int ihl = min_t(int, ip_hdrlen(pkt), skb_headlen(pkt));
 	struct sock *mroute_sk;
 	struct igmphdr *igmp;
 	struct igmpmsg *msg;

-- 
2.47.3