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>
static struct fib6_table *fib6_alloc_table(struct net *net, u32 id)
{
	struct fib6_table *table;

	table = kzalloc(sizeof(*table), GFP_ATOMIC);
	if (table) {
		table->tb6_id = id;
		table->tb6_root.leaf = net->ipv6.ip6_null_entry;
		table->tb6_root.fn_flags = RTN_ROOT | RTN_TL_ROOT | RTN_RTINFO;
		inet_peer_base_init(&table->tb6_peers);
	}

	return table;
}

static struct fib6_table *fib6_alloc_table(struct net *net, u32 id)
{
	struct fib6_table *table;

	table = kzalloc(sizeof(*table), GFP_ATOMIC);
	if (table) {
		table->tb6_id = id;
		table->tb6_root.leaf = net->ipv6.ip6_null_entry;
		table->tb6_root.fn_flags = RTN_ROOT | RTN_TL_ROOT | RTN_RTINFO;
		inet_peer_base_init(&table->tb6_peers);
	}

	return table;
}
