When CONFIG_IP_MULTIPLE_TABLES is enabled but no rule is added,
fib_lookup() performs route lookup directly on two tables.

Since the first lookup does not properly bail out, the result
of an error route in the merged local/main table could be
overwritten by another route in the default table:

  # unshare -n
  # ip link set lo up
  # ip route add 192.168.0.0/24 dev lo table 253
  # ip route add unreachable 192.168.0.0/24
  # ip route get 192.168.0.1
  192.168.0.1 dev lo table default uid 0
      cache <local>

Once a random rule is added, the error route is respected:

  # ip rule add table 0
  # ip rule del table 0
  # ip route get 192.168.0.1
  RTNETLINK answers: No route to host

Let's fix the inconsistent behaviour.

Fixes: f4530fa574df ("ipv4: Avoid overhead when no custom FIB rules are installed.")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
---
 include/net/ip_fib.h | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/include/net/ip_fib.h b/include/net/ip_fib.h
index a71a98505650..c63a3c4967ae 100644
--- a/include/net/ip_fib.h
+++ b/include/net/ip_fib.h

@@ -374,7 +374,7 @@ static inline int fib_lookup(struct net *net, struct flowi4 *flp,
 			     struct fib_result *res, unsigned int flags)
 {
 	struct fib_table *tb;
-	int err = -ENETUNREACH;
+	int err = -EAGAIN;
 
 	flags |= FIB_LOOKUP_NOREF;
 	if (net->ipv4.fib_has_custom_rules)

@@ -388,17 +388,16 @@ static inline int fib_lookup(struct net *net, struct flowi4 *flp,
 	if (tb)
 		err = fib_table_lookup(tb, flp, res, flags);
 
-	if (!err)
+	if (err != -EAGAIN)
 		goto out;
 
 	tb = rcu_dereference_rtnl(net->ipv4.fib_default);
 	if (tb)
 		err = fib_table_lookup(tb, flp, res, flags);
 
-out:
 	if (err == -EAGAIN)
 		err = -ENETUNREACH;
-
+out:
 	rcu_read_unlock();
 
 	return err;

-- 
2.55.0.rc0.786.g65d90a0328-goog