CVE-2026-23398

In the Linux kernel, the following vulnerability has been resolved:

icmp: fix NULL pointer dereference in icmp_tag_validation()

icmp_tag_validation() unconditionally dereferences the result of
rcu_dereference(inet_protos[proto]) without checking for NULL.
The inet_protos[] array is sparse -- only about 15 of 256 protocol
numbers have registered handlers. When ip_no_pmtu_disc is set to 3
(hardened PMTU mode) and the kernel receives an ICMP Fragmentation
Needed error with a quoted inner IP header containing an unregistered
protocol number, the NULL dereference causes a kernel panic in
softirq context.

 Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] SMP KASAN NOPTI
 KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
 RIP: 0010:icmp_unreach (net/ipv4/icmp.c:1085 net/ipv4/icmp.c:1143)
 Call Trace:
  <IRQ>
  icmp_rcv (net/ipv4/icmp.c:1527)
  ip_protocol_deliver_rcu (net/ipv4/ip_input.c:207)
  ip_local_deliver_finish (net/ipv4/ip_input.c:242)
  ip_local_deliver (net/ipv4/ip_input.c:262)
  ip_rcv (net/ipv4/ip_input.c:573)
  __netif_receive_skb_one_core (net/core/dev.c:6164)
  process_backlog (net/core/dev.c:6628)
  handle_softirqs (kernel/softirq.c:561)
  </IRQ>

Add a NULL check before accessing icmp_strict_tag_validation. If the
protocol has no registered handler, return false since it cannot
perform strict tag validation.