CVE-2024-56703: In the Linux kernel, the following vulnerability has been resolved:  ipv6: Fix soft lockups in fib6_select_path under high next hop churn  Soft lockups have been observed on a cluster of Linux-based edge routers located in a highly dynamic environment. Using the `bird` service, these routers continuously update BGP-advertised routes due to frequently changing nexthop destinations, while also managing significant IPv6 traffic. The lockups occur during the traversal of the multipath circular linked-list in the `fib6_select_path` function, particularly while iterating through the siblings in the list. The issue typically arises when the nodes of the linked list are unexpectedly deleted concurrently on a different core—indicated by their 'next' and 'previous' elements pointing back to the node itself and their reference count dropping to zero. This results in an infinite loop, leading to a soft lockup that triggers a system panic via the watchdog timer.  Apply RCU primitives in the problematic code sections to resolve the issue. Where necessary, update the references to fib6_siblings to annotate or use the RCU APIs.  Include a test script that reproduces the issue. The script periodically updates the routing table while generating a heavy load of outgoing IPv6 traffic through multiple iperf3 clients. It consistently induces infinite soft lockups within a couple of minutes.  Kernel log:   0 [ffffbd13003e8d30] machine_kexec at ffffffff8ceaf3eb  1 [ffffbd13003e8d90] __crash_kexec at ffffffff8d0120e3  2 [ffffbd13003e8e58] panic at ffffffff8cef65d4  3 [ffffbd13003e8ed8] watchdog_timer_fn at ffffffff8d05cb03  4 [ffffbd13003e8f08] __hrtimer_run_queues at ffffffff8cfec62f  5 [ffffbd13003e8f70] hrtimer_interrupt at ffffffff8cfed756  6 [ffffbd13003e8fd0] __sysvec_apic_timer_interrupt at ffffffff8cea01af  7 [ffffbd13003e8ff0] sysvec_apic_timer_interrupt at ffffffff8df1b83d -- <IRQ stack> --  8 [ffffbd13003d3708] asm_sysvec_apic_timer_interrupt at ffffffff8e000ecb     [exception RIP: fib6_select_path+299]     RIP: ffffffff8ddafe7b  RSP: ffffbd13003d37b8  RFLAGS: 00000287     RAX: ffff975850b43600  RBX: ffff975850b40200  RCX: 0000000000000000     RDX: 000000003fffffff  RSI: 0000000051d383e4  RDI: ffff975850b43618     RBP: ffffbd13003d3800   R8: 0000000000000000   R9: ffff975850b40200     R10: 0000000000000000  R11: 0000000000000000  R12: ffffbd13003d3830     R13: ffff975850b436a8  R14: ffff975850b43600  R15: 0000000000000007     ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018  9 [ffffbd13003d3808] ip6_pol_route at ffffffff8ddb030c 10 [ffffbd13003d3888] ip6_pol_route_input at ffffffff8ddb068c 11 [ffffbd13003d3898] fib6_rule_lookup at ffffffff8ddf02b5 12 [ffffbd13003d3928] ip6_route_input at ffffffff8ddb0f47 13 [ffffbd13003d3a18] ip6_rcv_finish_core.constprop.0 at ffffffff8dd950d0 14 [ffffbd13003d3a30] ip6_list_rcv_finish.constprop.0 at ffffffff8dd96274 15 [ffffbd13003d3a98] ip6_sublist_rcv at ffffffff8dd96474 16 [ffffbd13003d3af8] ipv6_list_rcv at ffffffff8dd96615 17 [ffffbd13003d3b60] __netif_receive_skb_list_core at ffffffff8dc16fec 18 [ffffbd13003d3be0] netif_receive_skb_list_internal at ffffffff8dc176b3 19 [ffffbd13003d3c50] napi_gro_receive at ffffffff8dc565b9 20 [ffffbd13003d3c80] ice_receive_skb at ffffffffc087e4f5 [ice] 21 [ffffbd13003d3c90] ice_clean_rx_irq at ffffffffc0881b80 [ice] 22 [ffffbd13003d3d20] ice_napi_poll at ffffffffc088232f [ice] 23 [ffffbd13003d3d80] __napi_poll at ffffffff8dc18000 24 [ffffbd13003d3db8] net_rx_action at ffffffff8dc18581 25 [ffffbd13003d3e40] __do_softirq at ffffffff8df352e9 26 [ffffbd13003d3eb0] run_ksoftirqd at ffffffff8ceffe47 27 [ffffbd13003d3ec0] smpboot_thread_fn at ffffffff8cf36a30 28 [ffffbd13003d3ee8] kthread at ffffffff8cf2b39f 29 [ffffbd13003d3f28] ret_from_fork at ffffffff8ce5fa64 30 [ffffbd13003d3f50] ret_from_fork_asm at ffffffff8ce03cbb