[PATCH net] ipv6: Fix a potential NPD in cleanup_prefix_route()

[Ido Schimmel <idosch@nvidia.com>]

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)

Fixes: 5eb902b8e719 ("net/ipv6: Remove expired routes with a separated list of routes.")
Reported-by: Ji'an Zhou <eilaimemedsnaimel@gmail.com>
Reviewed-by: David Ahern <dahern@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
---
 net/ipv6/addrconf.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c
index bb84a78b80f6..c9e5d3e48ab9 100644
--- a/net/ipv6/addrconf.c
+++ b/net/ipv6/addrconf.c
@@ -1265,6 +1265,7 @@ static void
 cleanup_prefix_route(struct inet6_ifaddr *ifp, unsigned long expires,
 		     bool del_rt, bool del_peer)
 {
+	struct net *net = dev_net(ifp->idev->dev);
 	struct fib6_table *table;
 	struct fib6_info *f6i;
 
@@ -1273,9 +1274,10 @@ cleanup_prefix_route(struct inet6_ifaddr *ifp, unsigned long expires,
 					ifp->idev->dev, 0, RTF_DEFAULT, true);
 	if (f6i) {
 		if (del_rt)
-			ip6_del_rt(dev_net(ifp->idev->dev), f6i, false);
+			ip6_del_rt(net, f6i, false);
 		else {
-			if (!(f6i->fib6_flags & RTF_EXPIRES)) {
+			if (f6i != net->ipv6.fib6_null_entry &&
+			    !(f6i->fib6_flags & RTF_EXPIRES)) {
 				table = f6i->fib6_table;
 				spin_lock_bh(&table->tb6_lock);
 
-- 
2.54.0