ipv6: fix route selection if kernel is not compiled with CONFIG_IPV6_ROUTER_PREF

This is a follow-up patch to 3630d40067a21d4dfbadc6002bb469ce26ac5d52
("ipv6: rt6_check_neigh should successfully verify neigh if no NUD
information are available").

Since the removal of rt->n in rt6_info we can end up with a dst ==
NULL in rt6_check_neigh. In case the kernel is not compiled with
CONFIG_IPV6_ROUTER_PREF we should also select a route with unkown
NUD state but we must not avoid doing round robin selection on routes
with the same target. So introduce and pass down a boolean ``do_rr'' to
indicate when we should update rt->rr_ptr. As soon as no route is valid
we do backtracking and do a lookup on a higher level in the fib trie.

v2:
a) Improved rt6_check_neigh logic (no need to create neighbour there)
   and documented return values.

v3:
a) Introduce enum rt6_nud_state to get rid of the magic numbers
   (thanks to David Miller).
b) Update and shorten commit message a bit to actualy reflect
   the source.

Reported-by: Pierre Emeriaud <petrus.lt@gmail.com>
Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 40 insertions, 23 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 5b127e09c224..a8c891aa2464 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -65,6 +65,12 @@
 #include <linux/sysctl.h>
 #endif
 
+enum rt6_nud_state {
+        RT6_NUD_FAIL_HARD = -2,
+        RT6_NUD_FAIL_SOFT = -1,
+        RT6_NUD_SUCCEED = 1
+};
+
 static struct rt6_info *ip6_rt_copy(struct rt6_info *ort,
                                     const struct in6_addr *dest);
 static struct dst_entry *ip6_dst_check(struct dst_entry *dst, u32 cookie);
@@ -531,28 +537,29 @@ static inline int rt6_check_dev(struct rt6_info *rt, int oif)
         return 0;
 }
 
-static inline bool rt6_check_neigh(struct rt6_info *rt)
+static inline enum rt6_nud_state rt6_check_neigh(struct rt6_info *rt)
 {
         struct neighbour *neigh;
-        bool ret = false;
+        enum rt6_nud_state ret = RT6_NUD_FAIL_HARD;
 
         if (rt->rt6i_flags & RTF_NONEXTHOP ||
             !(rt->rt6i_flags & RTF_GATEWAY))
-                return true;
+                return RT6_NUD_SUCCEED;
 
         rcu_read_lock_bh();
         neigh = __ipv6_neigh_lookup_noref(rt->dst.dev, &rt->rt6i_gateway);
         if (neigh) {
                 read_lock(&neigh->lock);
                 if (neigh->nud_state & NUD_VALID)
-                        ret = true;
+                        ret = RT6_NUD_SUCCEED;
 #ifdef CONFIG_IPV6_ROUTER_PREF
                 else if (!(neigh->nud_state & NUD_FAILED))
-                        ret = true;
+                        ret = RT6_NUD_SUCCEED;
 #endif
                 read_unlock(&neigh->lock);
-        } else if (IS_ENABLED(CONFIG_IPV6_ROUTER_PREF)) {
-                ret = true;
+        } else {
+                ret = IS_ENABLED(CONFIG_IPV6_ROUTER_PREF) ?
+                      RT6_NUD_SUCCEED : RT6_NUD_FAIL_SOFT;
         }
         rcu_read_unlock_bh();
 
@@ -566,43 +573,52 @@ static int rt6_score_route(struct rt6_info *rt, int oif,
 
         m = rt6_check_dev(rt, oif);
         if (!m && (strict & RT6_LOOKUP_F_IFACE))
-                return -1;
+                return RT6_NUD_FAIL_HARD;
 #ifdef CONFIG_IPV6_ROUTER_PREF
         m |= IPV6_DECODE_PREF(IPV6_EXTRACT_PREF(rt->rt6i_flags)) << 2;
 #endif
-        if (!rt6_check_neigh(rt) && (strict & RT6_LOOKUP_F_REACHABLE))
-                return -1;
+        if (strict & RT6_LOOKUP_F_REACHABLE) {
+                int n = rt6_check_neigh(rt);
+                if (n < 0)
+                        return n;
+        }
         return m;
 }
 
 static struct rt6_info *find_match(struct rt6_info *rt, int oif, int strict,
-                                   int *mpri, struct rt6_info *match)
+                                   int *mpri, struct rt6_info *match,
+                                   bool *do_rr)
 {
         int m;
+        bool match_do_rr = false;
 
         if (rt6_check_expired(rt))
                 goto out;
 
         m = rt6_score_route(rt, oif, strict);
-        if (m < 0)
+        if (m == RT6_NUD_FAIL_SOFT && !IS_ENABLED(CONFIG_IPV6_ROUTER_PREF)) {
+                match_do_rr = true;
+                m = 0; /* lowest valid score */
+        } else if (m < 0) {
                 goto out;
+        }
+
+        if (strict & RT6_LOOKUP_F_REACHABLE)
+                rt6_probe(rt);
 
         if (m > *mpri) {
-                if (strict & RT6_LOOKUP_F_REACHABLE)
-                        rt6_probe(match);
+                *do_rr = match_do_rr;
                 *mpri = m;
                 match = rt;
-        } else if (strict & RT6_LOOKUP_F_REACHABLE) {
-                rt6_probe(rt);
         }
-
 out:
         return match;
 }
 
 static struct rt6_info *find_rr_leaf(struct fib6_node *fn,
                                      struct rt6_info *rr_head,
-                                     u32 metric, int oif, int strict)
+                                     u32 metric, int oif, int strict,
+                                     bool *do_rr)
 {
         struct rt6_info *rt, *match;
         int mpri = -1;
@@ -610,10 +626,10 @@ static struct rt6_info *find_rr_leaf(struct fib6_node *fn,
         match = NULL;
         for (rt = rr_head; rt && rt->rt6i_metric == metric;
              rt = rt->dst.rt6_next)
-                match = find_match(rt, oif, strict, &mpri, match);
+                match = find_match(rt, oif, strict, &mpri, match, do_rr);
         for (rt = fn->leaf; rt && rt != rr_head && rt->rt6i_metric == metric;
              rt = rt->dst.rt6_next)
-                match = find_match(rt, oif, strict, &mpri, match);
+                match = find_match(rt, oif, strict, &mpri, match, do_rr);
 
         return match;
 }
@@ -622,15 +638,16 @@ static struct rt6_info *rt6_select(struct fib6_node *fn, int oif, int strict)
 {
         struct rt6_info *match, *rt0;
         struct net *net;
+        bool do_rr = false;
 
         rt0 = fn->rr_ptr;
         if (!rt0)
                 fn->rr_ptr = rt0 = fn->leaf;
 
-        match = find_rr_leaf(fn, rt0, rt0->rt6i_metric, oif, strict);
+        match = find_rr_leaf(fn, rt0, rt0->rt6i_metric, oif, strict,
+                             &do_rr);
 
-        if (!match &&
-            (strict & RT6_LOOKUP_F_REACHABLE)) {
+        if (do_rr) {
                 struct rt6_info *next = rt0->dst.rt6_next;
 
                 /* no entries matched; do round-robin */