net: fib: fib6_add: fix potential NULL pointer dereference

When the kernel is compiled with CONFIG_IPV6_SUBTREES, and we return
with an error in fn = fib6_add_1(), then error codes are encoded into
the return pointer e.g. ERR_PTR(-ENOENT). In such an error case, we
write the error code into err and jump to out, hence enter the if(err)
condition. Now, if CONFIG_IPV6_SUBTREES is enabled, we check for:

  if (pn != fn && pn->leaf == rt)
    ...
  if (pn != fn && !pn->leaf && !(pn->fn_flags & RTN_RTINFO))
    ...

Since pn is NULL and fn is f.e. ERR_PTR(-ENOENT), then pn != fn
evaluates to true and causes a NULL-pointer dereference on further
checks on pn. Fix it, by setting both NULL in error case, so that
pn != fn already evaluates to false and no further dereference
takes place.

This was first correctly implemented in 4a287eba2 ("IPv6 routing,
NLM_F_* flag support: REPLACE and EXCL flags support, warn about
missing CREATE flag"), but the bug got later on introduced by
188c517a0 ("ipv6: return errno pointers consistently for fib6_add_1()").

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: Lin Ming <mlin@ss.pku.edu.cn>
Cc: Matti Vaittinen <matti.vaittinen@nsn.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Matti Vaittinen <matti.vaittinen@nsn.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
void fib6_clean_all(struct net *net, int (*func)(struct rt6_info *, void *arg),
		    int prune, void *arg)
{
	struct fib6_table *table;
	struct hlist_head *head;
	unsigned int h;

	rcu_read_lock();
	for (h = 0; h < FIB6_TABLE_HASHSZ; h++) {
		head = &net->ipv6.fib_table_hash[h];
		hlist_for_each_entry_rcu(table, head, tb6_hlist) {
			write_lock_bh(&table->tb6_lock);
			fib6_clean_tree(net, &table->tb6_root,
					func, prune, arg);
			write_unlock_bh(&table->tb6_lock);
		}
	}
	rcu_read_unlock();
}

void fib6_clean_all(struct net *net, int (*func)(struct rt6_info *, void *arg),
		    int prune, void *arg)
{
	struct fib6_table *table;
	struct hlist_head *head;
	unsigned int h;

	rcu_read_lock();
	for (h = 0; h < FIB6_TABLE_HASHSZ; h++) {
		head = &net->ipv6.fib_table_hash[h];
		hlist_for_each_entry_rcu(table, head, tb6_hlist) {
			write_lock_bh(&table->tb6_lock);
			fib6_clean_tree(net, &table->tb6_root,
					func, prune, arg);
			write_unlock_bh(&table->tb6_lock);
		}
	}
	rcu_read_unlock();
}
