[PATCH net] ipv6: ignore link state in gateway check

[Yaroslav Dudkov <aroslavdudkov622@gmail.com>]

IPv6 route cannot be added in table when all
following conditions are true:
- gateway is not link-local address
- ignore_routes_with_linkdown=1 on device
- nexthop has no carrier
- gateway is not present or covered by any route in table already
Also multipath routes addition is invalidated by kernel even
if only one of added nexthops satisfies all above conditions.

Reproducer:

  ip netns add t
  ip -n t link add va type veth peer name vb
  ip -n t link set va up
  ip -n t addr add fd00::1/64 dev va nodad
  ip netns exec t sysctl -w \
    net.ipv6.conf.va.ignore_routes_with_linkdown=1
  ip -n t -6 route add 2001:db8::/64 via fd00::2 dev va
  # added
  ip -n t -6 route add 2001:db8::/64 table 100 via fd00::2 dev va
  # RTNETLINK answers: No route to host
  ip -n t -6 route show table all
  ip netns del t

Route should be added because:
- RT6_LOOKUP_F_IGNORE_LINKSTATE in commit d5d32e4b7668
("net: ipv6: Do not consider link state for nexthop validation")
was added for this
- the same route is valid in main table (when no specifying table id
while adding route)
- fib6_nh_init() initializes nexthops with no carrier
- IPv4 already does this (fib_check_nh_v4_gw() ignores link state
using the FIB_LOOKUP_IGNORE_LINKSTATE flag)

Before commit b2c709cce682 ("ipv6: Convert gateway validation to
use fib6_info") rt6_lookup() was used for checking if nexthop is
valid, rt6_lookup called rt6_device_match() which does not check the
link state. After change rt6_lookup() to fib6_lookup()
find_match() under fib6_lookup() started to invalidate linkdown
devices, because RT6_LOOKUP_F_IGNORE_LINKSTATE is not passed to it.

Adding RT6_LOOKUP_F_IGNORE_LINKSTATE to flags for ip6_route_check_nh()
solves the problem. RT6_LOOKUP_F_IGNORE_LINKSTATE is already used
inside the first search ip6_nh_lookup_table() and fixes the
bug in second search fib6_lookup(). Also RT6_LOOKUP_F_IGNORE_LINKSTATE
isn't read inside fib6_select_path().

Fixes: b2c709cce682 ("ipv6: Convert gateway validation to use fib6_info")
Signed-off-by: Yaroslav Dudkov <aroslavdudkov622@gmail.com>
---
IPv4 example of this situation:
  ip netns add t
  ip -n t link add va type veth peer name vb
  ip -n t link set va up
  ip -n t addr add 192.168.0.1/24 dev va
  ip netns exec t sysctl -w \
    net.ipv4.conf.va.ignore_routes_with_linkdown=1
  ip -n t route add 192.0.2.0/24 table 100 via 192.168.0.2 dev va
  # added
  ip -n t route show table all
  ip netns del t

Also when adding RT6_LOOKUP_F_IGNORE_LINKSTATE to flags of
ip6_route_check_nh() makes adding this flag inside of
ip6_nh_lookup_table() unnecessary for this call, making another call in
ip6_route_check_nh_onlink() with flags=0 the only place where it is
needed.
---
 net/ipv6/route.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 94e5ef1c4f83..95d7b5b10e51 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -3426,7 +3426,7 @@ static int ip6_route_check_nh(struct net *net,
 {
 	const struct in6_addr *gw_addr = &cfg->fc_gateway;
 	struct net_device *dev = _dev ? *_dev : NULL;
-	int flags = RT6_LOOKUP_F_IFACE;
+	int flags = RT6_LOOKUP_F_IFACE | RT6_LOOKUP_F_IGNORE_LINKSTATE;
 	struct fib6_result res = {};
 	int err = -EHOSTUNREACH;
 
-- 
2.54.0