ipv6: release nexthop on device removal

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: ipv6: release nexthop on device removal The CI is hitting some aperiodic hangup at device removal time in the pmtu.sh self-test: unregister_netdevice: waiting for veth_A-R1 to become free. Usage count = 6 ref_tracker: veth_A-R1@ffff888013df15d8 has 1/5 users at dst_init+0x84/0x4a0 dst_alloc+0x97/0x150 ip6_dst_alloc+0x23/0x90 ip6_rt_pcpu_alloc+0x1e6/0x520 ip6_pol_route+0x56f/0x840 fib6_rule_lookup+0x334/0x630 ip6_route_output_flags+0x259/0x480 ip6_dst_lookup_tail.constprop.0+0x5c2/0x940 ip6_dst_lookup_flow+0x88/0x190 udp_tunnel6_dst_lookup+0x2a7/0x4c0 vxlan_xmit_one+0xbde/0x4a50 [vxlan] vxlan_xmit+0x9ad/0xf20 [vxlan] dev_hard_start_xmit+0x10e/0x360 __dev_queue_xmit+0xf95/0x18c0 arp_solicit+0x4a2/0xe00 neigh_probe+0xaa/0xf0 While the first suspect is the dst_cache, explicitly tracking the dst owing the last device reference via probes proved such dst is held by the nexthop in the originating fib6_info. Similar to commit f5b51fe804ec ("ipv6: route: purge exception on removal"), we need to explicitly release the originating fib info when disconnecting a to-be-removed device from a live ipv6 dst: move the fib6_info cleanup into ip6_dst_ifdown(). Tested running: ./pmtu.sh cleanup_ipv6_exception in a tight loop for more than 400 iterations with no spat, running an unpatched kernel I observed a splat every ~10 iterations.

References

• https://git.kernel.org/stable/c/77aa9855a878fb43f547ddfbda3127a1e88ad31a
• https://git.kernel.org/stable/c/b2f26a27ea3f72f75d18330f76f5d1007c791848
• https://git.kernel.org/stable/c/43e25adc80269f917d2a195f0d59f74cdd182955
• https://git.kernel.org/stable/c/a3c3f8a4d025acc8c857246ec2b812c59102487a
• https://git.kernel.org/stable/c/0e4c6faaef8a24b762a24ffb767280e263ef8e10
• https://git.kernel.org/stable/c/eb02688c5c45c3e7af7e71f036a7144f5639cbfe