ipv6: Fix a potential NPD in cleanup_prefix_route()

Summary

In the Linux kernel, the following vulnerability has been resolved: ipv6: Fix a potential NPD in cleanup_prefix_route() addrconf_get_prefix_route() can return the fib6_null_entry sentinel entry which has a NULL fib6_table pointer. Therefore, before setting the route's expiration time, check that we are not working with this entry, as otherwise a NPD will be triggered [1]. Note that the other callers of addrconf_get_prefix_route() are not susceptible to this bug: 1. addrconf_prefix_rcv(): Requests a route with the 'RTF_ADDRCONF | RTF_PREFIX_RT' flags which are not set on fib6_null_entry. 2. modify_prefix_route(): Fixed by commit a747e02430df ("ipv6: avoid possible NULL deref in modify_prefix_route()"). 3. __ipv6_ifa_notify(): Calls ip6_del_rt() which specifically checks for fib6_null_entry and returns an error. [1] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000006: 0000 [#1] SMP KASAN KASAN: null-ptr-deref in range [0x0000000000000030-0x0000000000000037] [...] Call Trace: <TASK> __kasan_check_byte (mm/kasan/common.c:573) lock_acquire.part.0 (kernel/locking/lockdep.c:5842 (discriminator 1)) _raw_spin_lock_bh (kernel/locking/spinlock.c:182 (discriminator 1)) cleanup_prefix_route (net/ipv6/addrconf.c:1280) ipv6_del_addr (net/ipv6/addrconf.c:1342) inet6_addr_del.isra.0 (net/ipv6/addrconf.c:3119) inet6_rtm_deladdr (net/ipv6/addrconf.c:4812) rtnetlink_rcv_msg (net/core/rtnetlink.c:6997) netlink_rcv_skb (net/netlink/af_netlink.c:2555) netlink_unicast (net/netlink/af_netlink.c:1344) netlink_sendmsg (net/netlink/af_netlink.c:1899) __sock_sendmsg (net/socket.c:802 (discriminator 4)) ____sys_sendmsg (net/socket.c:2698) ___sys_sendmsg (net/socket.c:2752) __sys_sendmsg (net/socket.c:2784) do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)