ipv6: remove null_entry before adding default route

In the current code, when creating a new fib6 table, tb6_root.leaf gets
initialized to net->ipv6.ip6_null_entry.
If a default route is being added with rt->rt6i_metric = 0xffffffff,
fib6_add() will add this route after net->ipv6.ip6_null_entry. As
null_entry is shared, it could cause problem.

In order to fix it, set fn->leaf to NULL before calling
fib6_add_rt2node() when trying to add the first default route.
And reset fn->leaf to null_entry when adding fails or when deleting the
last default route.

syzkaller reported the following issue which is fixed by this commit:

WARNING: suspicious RCU usage
4.15.0-rc5+ #171 Not tainted
-----------------------------
net/ipv6/ip6_fib.c:1702 suspicious rcu_dereference_protected() usage!

other info that might help us debug this:

rcu_scheduler_active = 2, debug_locks = 1
4 locks held by swapper/0/0:
 #0:  ((&net->ipv6.ip6_fib_timer)){+.-.}, at: [<00000000d43f631b>] lockdep_copy_map include/linux/lockdep.h:178 [inline]
 #0:  ((&net->ipv6.ip6_fib_timer)){+.-.}, at: [<00000000d43f631b>] call_timer_fn+0x1c6/0x820 kernel/time/timer.c:1310
 #1:  (&(&net->ipv6.fib6_gc_lock)->rlock){+.-.}, at: [<000000002ff9d65c>] spin_lock_bh include/linux/spinlock.h:315 [inline]
 #1:  (&(&net->ipv6.fib6_gc_lock)->rlock){+.-.}, at: [<000000002ff9d65c>] fib6_run_gc+0x9d/0x3c0 net/ipv6/ip6_fib.c:2007
 #2:  (rcu_read_lock){....}, at: [<0000000091db762d>] __fib6_clean_all+0x0/0x3a0 net/ipv6/ip6_fib.c:1560
 #3:  (&(&tb->tb6_lock)->rlock){+.-.}, at: [<000000009e503581>] spin_lock_bh include/linux/spinlock.h:315 [inline]
 #3:  (&(&tb->tb6_lock)->rlock){+.-.}, at: [<000000009e503581>] __fib6_clean_all+0x1d0/0x3a0 net/ipv6/ip6_fib.c:1948

stack backtrace:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.15.0-rc5+ #171
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 lockdep_rcu_suspicious+0x123/0x170 kernel/locking/lockdep.c:4585
 fib6_del+0xcaa/0x11b0 net/ipv6/ip6_fib.c:1701
 fib6_clean_node+0x3aa/0x4f0 net/ipv6/ip6_fib.c:1892
 fib6_walk_continue+0x46c/0x8a0 net/ipv6/ip6_fib.c:1815
 fib6_walk+0x91/0xf0 net/ipv6/ip6_fib.c:1863
 fib6_clean_tree+0x1e6/0x340 net/ipv6/ip6_fib.c:1933
 __fib6_clean_all+0x1f4/0x3a0 net/ipv6/ip6_fib.c:1949
 fib6_clean_all net/ipv6/ip6_fib.c:1960 [inline]
 fib6_run_gc+0x16b/0x3c0 net/ipv6/ip6_fib.c:2016
 fib6_gc_timer_cb+0x20/0x30 net/ipv6/ip6_fib.c:2033
 call_timer_fn+0x228/0x820 kernel/time/timer.c:1320
 expire_timers kernel/time/timer.c:1357 [inline]
 __run_timers+0x7ee/0xb70 kernel/time/timer.c:1660
 run_timer_softirq+0x4c/0xb0 kernel/time/timer.c:1686
 __do_softirq+0x2d7/0xb85 kernel/softirq.c:285
 invoke_softirq kernel/softirq.c:365 [inline]
 irq_exit+0x1cc/0x200 kernel/softirq.c:405
 exiting_irq arch/x86/include/asm/apic.h:540 [inline]
 smp_apic_timer_interrupt+0x16b/0x700 arch/x86/kernel/apic/apic.c:1052
 apic_timer_interrupt+0xa9/0xb0 arch/x86/entry/entry_64.S:904
 </IRQ>
Reported-by: Nsyzbot <syzkaller@googlegroups.com>
Fixes: 66f5d6ce ("ipv6: replace rwlock with rcu and spinlock in fib6_table")
Signed-off-by: NWei Wang <weiwan@google.com>
Acked-by: NMartin KaFai Lau <kafai@fb.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/ip6_fib.c

@@ -640,6 +640,11 @@ static struct fib6_node *fib6_add_1(struct net *net,
 			if (!(fn->fn_flags & RTN_RTINFO)) {
 				RCU_INIT_POINTER(fn->leaf, NULL);
 				rt6_release(leaf);
+			/* remove null_entry in the root node */
+			} else if (fn->fn_flags & RTN_TL_ROOT &&
+				   rcu_access_pointer(fn->leaf) ==
+				   net->ipv6.ip6_null_entry) {
+				RCU_INIT_POINTER(fn->leaf, NULL);
 			}
 
 			return fn;
@@ -1270,13 +1275,17 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt,
 	return err;
 
 failure:
-	/* fn->leaf could be NULL if fn is an intermediate node and we
-	 * failed to add the new route to it in both subtree creation
-	 * failure and fib6_add_rt2node() failure case.
-	 * In both cases, fib6_repair_tree() should be called to fix
-	 * fn->leaf.
+	/* fn->leaf could be NULL and fib6_repair_tree() needs to be called if:
+	 * 1. fn is an intermediate node and we failed to add the new
+	 * route to it in both subtree creation failure and fib6_add_rt2node()
+	 * failure case.
+	 * 2. fn is the root node in the table and we fail to add the first
+	 * default route to it.
 	 */
-	if (fn && !(fn->fn_flags & (RTN_RTINFO|RTN_ROOT)))
+	if (fn &&
+	    (!(fn->fn_flags & (RTN_RTINFO|RTN_ROOT)) ||
+	     (fn->fn_flags & RTN_TL_ROOT &&
+	      !rcu_access_pointer(fn->leaf))))
 		fib6_repair_tree(info->nl_net, table, fn);
 	/* Always release dst as dst->__refcnt is guaranteed
 	 * to be taken before entering this function
@@ -1531,6 +1540,12 @@ static struct fib6_node *fib6_repair_tree(struct net *net,
 	struct fib6_walker *w;
 	int iter = 0;
 
+	/* Set fn->leaf to null_entry for root node. */
+	if (fn->fn_flags & RTN_TL_ROOT) {
+		rcu_assign_pointer(fn->leaf, net->ipv6.ip6_null_entry);
+		return fn;
+	}
+
 	for (;;) {
 		struct fib6_node *fn_r = rcu_dereference_protected(fn->right,
 					    lockdep_is_held(&table->tb6_lock));
@@ -1685,10 +1700,15 @@ static void fib6_del_route(struct fib6_table *table, struct fib6_node *fn,
 	}
 	read_unlock(&net->ipv6.fib6_walker_lock);
 
-	/* If it was last route, expunge its radix tree node */
+	/* If it was last route, call fib6_repair_tree() to:
+	 * 1. For root node, put back null_entry as how the table was created.
+	 * 2. For other nodes, expunge its radix tree node.
+	 */
 	if (!rcu_access_pointer(fn->leaf)) {
-		fn->fn_flags &= ~RTN_RTINFO;
-		net->ipv6.rt6_stats->fib_route_nodes--;
+		if (!(fn->fn_flags & RTN_TL_ROOT)) {
+			fn->fn_flags &= ~RTN_RTINFO;
+			net->ipv6.rt6_stats->fib_route_nodes--;
+		}
 		fn = fib6_repair_tree(net, table, fn);
 	}