ipv4: lock mtu in fnhe when received PMTU < net.ipv4.route.min_pmtu

Prior to the rework of PMTU information storage in commit
2c8cec5c10bc ("ipv4: Cache learned PMTU information in inetpeer."),
when a PMTU event advertising a PMTU smaller than
net.ipv4.route.min_pmtu was received, we would disable setting the DF
flag on packets by locking the MTU metric, and set the PMTU to
net.ipv4.route.min_pmtu.

Since then, we don't disable DF, and set PMTU to
net.ipv4.route.min_pmtu, so the intermediate router that has this link
with a small MTU will have to drop the packets.

This patch reestablishes pre-2.6.39 behavior by splitting
rtable->rt_pmtu into a bitfield with rt_mtu_locked and rt_pmtu.
rt_mtu_locked indicates that we shouldn't set the DF bit on that path,
and is checked in ip_dont_fragment().

One possible workaround is to set net.ipv4.route.min_pmtu to a value low
enough to accommodate the lowest MTU encountered.

Fixes: 2c8cec5c10bc ("ipv4: Cache learned PMTU information in inetpeer.")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 20 insertions, 7 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index f9dbb8cb66bf..299e247b2032 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -634,6 +634,7 @@ static inline u32 fnhe_hashfun(__be32 daddr)
 static void fill_route_from_fnhe(struct rtable *rt, struct fib_nh_exception *fnhe)
 {
         rt->rt_pmtu = fnhe->fnhe_pmtu;
+        rt->rt_mtu_locked = fnhe->fnhe_mtu_locked;
         rt->dst.expires = fnhe->fnhe_expires;
 
         if (fnhe->fnhe_gw) {
@@ -644,7 +645,7 @@ static void fill_route_from_fnhe(struct rtable *rt, struct fib_nh_exception *fnh
 }
 
 static void update_or_create_fnhe(struct fib_nh *nh, __be32 daddr, __be32 gw,
-                                  u32 pmtu, unsigned long expires)
+                                  u32 pmtu, bool lock, unsigned long expires)
 {
         struct fnhe_hash_bucket *hash;
         struct fib_nh_exception *fnhe;
@@ -681,8 +682,10 @@ static void update_or_create_fnhe(struct fib_nh *nh, __be32 daddr, __be32 gw,
                         fnhe->fnhe_genid = genid;
                 if (gw)
                         fnhe->fnhe_gw = gw;
-                if (pmtu)
+                if (pmtu) {
                         fnhe->fnhe_pmtu = pmtu;
+                        fnhe->fnhe_mtu_locked = lock;
+                }
                 fnhe->fnhe_expires = max(1UL, expires);
                 /* Update all cached dsts too */
                 rt = rcu_dereference(fnhe->fnhe_rth_input);
@@ -706,6 +709,7 @@ static void update_or_create_fnhe(struct fib_nh *nh, __be32 daddr, __be32 gw,
                 fnhe->fnhe_daddr = daddr;
                 fnhe->fnhe_gw = gw;
                 fnhe->fnhe_pmtu = pmtu;
+                fnhe->fnhe_mtu_locked = lock;
                 fnhe->fnhe_expires = expires;
 
                 /* Exception created; mark the cached routes for the nexthop
@@ -787,7 +791,8 @@ static void __ip_do_redirect(struct rtable *rt, struct sk_buff *skb, struct flow
                                 struct fib_nh *nh = &FIB_RES_NH(res);
 
                                 update_or_create_fnhe(nh, fl4->daddr, new_gw,
-                                                0, jiffies + ip_rt_gc_timeout);
+                                                0, false,
+                                                jiffies + ip_rt_gc_timeout);
                         }
                         if (kill_route)
                                 rt->dst.obsolete = DST_OBSOLETE_KILL;
@@ -1009,15 +1014,18 @@ static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
 {
         struct dst_entry *dst = &rt->dst;
         struct fib_result res;
+        bool lock = false;
 
-        if (dst_metric_locked(dst, RTAX_MTU))
+        if (ip_mtu_locked(dst))
                 return;
 
         if (ipv4_mtu(dst) < mtu)
                 return;
 
-        if (mtu < ip_rt_min_pmtu)
+        if (mtu < ip_rt_min_pmtu) {
+                lock = true;
                 mtu = ip_rt_min_pmtu;
+        }
 
         if (rt->rt_pmtu == mtu &&
             time_before(jiffies, dst->expires - ip_rt_mtu_expires / 2))
@@ -1027,7 +1035,7 @@ static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
         if (fib_lookup(dev_net(dst->dev), fl4, &res, 0) == 0) {
                 struct fib_nh *nh = &FIB_RES_NH(res);
 
-                update_or_create_fnhe(nh, fl4->daddr, 0, mtu,
+                update_or_create_fnhe(nh, fl4->daddr, 0, mtu, lock,
                                       jiffies + ip_rt_mtu_expires);
         }
         rcu_read_unlock();
@@ -1280,7 +1288,7 @@ static unsigned int ipv4_mtu(const struct dst_entry *dst)
 
         mtu = READ_ONCE(dst->dev->mtu);
 
-        if (unlikely(dst_metric_locked(dst, RTAX_MTU))) {
+        if (unlikely(ip_mtu_locked(dst))) {
                 if (rt->rt_uses_gateway && mtu > 576)
                         mtu = 576;
         }
@@ -1521,6 +1529,7 @@ struct rtable *rt_dst_alloc(struct net_device *dev,
                 rt->rt_is_input = 0;
                 rt->rt_iif = 0;
                 rt->rt_pmtu = 0;
+                rt->rt_mtu_locked = 0;
                 rt->rt_gateway = 0;
                 rt->rt_uses_gateway = 0;
                 rt->rt_table_id = 0;
@@ -2546,6 +2555,7 @@ struct dst_entry *ipv4_blackhole_route(struct net *net, struct dst_entry *dst_or
                 rt->rt_is_input = ort->rt_is_input;
                 rt->rt_iif = ort->rt_iif;
                 rt->rt_pmtu = ort->rt_pmtu;
+                rt->rt_mtu_locked = ort->rt_mtu_locked;
 
                 rt->rt_genid = rt_genid_ipv4(net);
                 rt->rt_flags = ort->rt_flags;
@@ -2648,6 +2658,8 @@ static int rt_fill_info(struct net *net,  __be32 dst, __be32 src, u32 table_id,
         memcpy(metrics, dst_metrics_ptr(&rt->dst), sizeof(metrics));
         if (rt->rt_pmtu && expires)
                 metrics[RTAX_MTU - 1] = rt->rt_pmtu;
+        if (rt->rt_mtu_locked && expires)
+                metrics[RTAX_LOCK - 1] |= BIT(RTAX_MTU);
         if (rtnetlink_put_metrics(skb, metrics) < 0)
                 goto nla_put_failure;
 
diff --git a/net/ipv4/xfrm4_policy.c b/net/ipv4/xfrm4_policy.c
index 8d33f7b311f4..fbebda67ac1b 100644
--- a/net/ipv4/xfrm4_policy.c
+++ b/net/ipv4/xfrm4_policy.c
@@ -100,6 +100,7 @@ static int xfrm4_fill_dst(struct xfrm_dst *xdst, struct net_device *dev,
         xdst->u.rt.rt_gateway = rt->rt_gateway;
         xdst->u.rt.rt_uses_gateway = rt->rt_uses_gateway;
         xdst->u.rt.rt_pmtu = rt->rt_pmtu;
+        xdst->u.rt.rt_mtu_locked = rt->rt_mtu_locked;
         xdst->u.rt.rt_table_id = rt->rt_table_id;
         INIT_LIST_HEAD(&xdst->u.rt.rt_uncached);
         rt_add_uncached_list(&xdst->u.rt);