ipv6: fix general protection fault in fib6_add()

In fib6_add(), pn could be NULL if fib6_add_1() failed to return a fib6
node. Checking pn != fn before accessing pn->leaf makes sure pn is not
NULL.
This fixes the following GPF reported by syzkaller:
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 0 PID: 3201 Comm: syzkaller001778 Not tainted 4.15.0-rc5+ #151
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:fib6_add+0x736/0x15a0 net/ipv6/ip6_fib.c:1244
RSP: 0018:ffff8801c7626a70 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000020 RCX: ffffffff84794465
RDX: 0000000000000004 RSI: ffff8801d38935f0 RDI: 0000000000000282
RBP: ffff8801c7626da0 R08: 1ffff10038ec4c35 R09: 0000000000000000
R10: ffff8801c7626c68 R11: 0000000000000000 R12: 00000000fffffffe
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000009
FS:  0000000000000000(0000) GS:ffff8801db200000(0063) knlGS:0000000009b70840
CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
CR2: 0000000020be1000 CR3: 00000001d585a006 CR4: 00000000001606f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 __ip6_ins_rt+0x6c/0x90 net/ipv6/route.c:1006
 ip6_route_multipath_add+0xd14/0x16c0 net/ipv6/route.c:3833
 inet6_rtm_newroute+0xdc/0x160 net/ipv6/route.c:3957
 rtnetlink_rcv_msg+0x733/0x1020 net/core/rtnetlink.c:4411
 netlink_rcv_skb+0x21e/0x460 net/netlink/af_netlink.c:2408
 rtnetlink_rcv+0x1c/0x20 net/core/rtnetlink.c:4423
 netlink_unicast_kernel net/netlink/af_netlink.c:1275 [inline]
 netlink_unicast+0x4e8/0x6f0 net/netlink/af_netlink.c:1301
 netlink_sendmsg+0xa4a/0xe60 net/netlink/af_netlink.c:1864
 sock_sendmsg_nosec net/socket.c:636 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:646
 sock_write_iter+0x31a/0x5d0 net/socket.c:915
 call_write_iter include/linux/fs.h:1772 [inline]
 do_iter_readv_writev+0x525/0x7f0 fs/read_write.c:653
 do_iter_write+0x154/0x540 fs/read_write.c:932
 compat_writev+0x225/0x420 fs/read_write.c:1246
 do_compat_writev+0x115/0x220 fs/read_write.c:1267
 C_SYSC_writev fs/read_write.c:1278 [inline]
 compat_SyS_writev+0x26/0x30 fs/read_write.c:1274
 do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
 do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
 entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:125

Reported-by: syzbot <syzkaller@googlegroups.com>
Fixes: 66f5d6ce53 ("ipv6: replace rwlock with rcu and spinlock in fib6_table")
Signed-off-by: Wei Wang <weiwan@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -1241,23 +1241,28 @@ out:
 		 * If fib6_add_1 has cleared the old leaf pointer in the
 		 * super-tree leaf node we have to find a new one for it.
 		 */
-		struct rt6_info *pn_leaf = rcu_dereference_protected(pn->leaf,
-					    lockdep_is_held(&table->tb6_lock));
-		if (pn != fn && pn_leaf == rt) {
-			pn_leaf = NULL;
-			RCU_INIT_POINTER(pn->leaf, NULL);
-			atomic_dec(&rt->rt6i_ref);
-		}
-		if (pn != fn && !pn_leaf && !(pn->fn_flags & RTN_RTINFO)) {
-			pn_leaf = fib6_find_prefix(info->nl_net, table, pn);
-#if RT6_DEBUG >= 2
-			if (!pn_leaf) {
-				WARN_ON(!pn_leaf);
-				pn_leaf = info->nl_net->ipv6.ip6_null_entry;
+		if (pn != fn) {
+			struct rt6_info *pn_leaf =
+				rcu_dereference_protected(pn->leaf,
+				    lockdep_is_held(&table->tb6_lock));
+			if (pn_leaf == rt) {
+				pn_leaf = NULL;
+				RCU_INIT_POINTER(pn->leaf, NULL);
+				atomic_dec(&rt->rt6i_ref);
 			}
+			if (!pn_leaf && !(pn->fn_flags & RTN_RTINFO)) {
+				pn_leaf = fib6_find_prefix(info->nl_net, table,
+							   pn);
+#if RT6_DEBUG >= 2
+				if (!pn_leaf) {
+					WARN_ON(!pn_leaf);
+					pn_leaf =
+					    info->nl_net->ipv6.ip6_null_entry;
+				}
 #endif
-			atomic_inc(&pn_leaf->rt6i_ref);
-			rcu_assign_pointer(pn->leaf, pn_leaf);
+				atomic_inc(&pn_leaf->rt6i_ref);
+				rcu_assign_pointer(pn->leaf, pn_leaf);
+			}
 		}
 #endif
 		goto failure;