net: ipv4: fix RCU races on dst refcounts

commit c6cffba4ffa2 (ipv4: Fix input route performance regression.)
added various fatal races with dst refcounts.

crashes happen on tcp workloads if routes are added/deleted at the same
time.

The dst_free() calls from free_fib_info_rcu() are clearly racy.

We need instead regular dst refcounting (dst_release()) and make
sure dst_release() is aware of RCU grace periods :

Add DST_RCU_FREE flag so that dst_release() respects an RCU grace period
before dst destruction for cached dst

Introduce a new inet_sk_rx_dst_set() helper, using atomic_inc_not_zero()
to make sure we dont increase a zero refcount (On a dst currently
waiting an rcu grace period before destruction)

rt_cache_route() must take a reference on the new cached route, and
release it if was not able to install it.

With this patch, my machines survive various benchmarks.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

7 files changed, 41 insertions, 29 deletions

diff --git a/net/core/dst.c b/net/core/dst.c
index 069d51d29414..d9e33ebe170f 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -258,6 +258,15 @@ again:
 }
 EXPORT_SYMBOL(dst_destroy);
 
+static void dst_rcu_destroy(struct rcu_head *head)
+{
+        struct dst_entry *dst = container_of(head, struct dst_entry, rcu_head);
+
+        dst = dst_destroy(dst);
+        if (dst)
+                __dst_free(dst);
+}
+
 void dst_release(struct dst_entry *dst)
 {
         if (dst) {
@@ -265,10 +274,14 @@ void dst_release(struct dst_entry *dst)
 
                 newrefcnt = atomic_dec_return(&dst->__refcnt);
                 WARN_ON(newrefcnt < 0);
-                if (unlikely(dst->flags & DST_NOCACHE) && !newrefcnt) {
-                        dst = dst_destroy(dst);
-                        if (dst)
-                                __dst_free(dst);
+                if (unlikely(dst->flags & (DST_NOCACHE | DST_RCU_FREE)) && !newrefcnt) {
+                        if (dst->flags & DST_RCU_FREE) {
+                                call_rcu_bh(&dst->rcu_head, dst_rcu_destroy);
+                        } else {
+                                dst = dst_destroy(dst);
+                                if (dst)
+                                        __dst_free(dst);
+                        }
                 }
         }
 }
@@ -320,11 +333,14 @@ EXPORT_SYMBOL(__dst_destroy_metrics_generic);
  */
 void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
 {
+        bool hold;
+
         WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
         /* If dst not in cache, we must take a reference, because
          * dst_release() will destroy dst as soon as its refcount becomes zero
          */
-        if (unlikely(dst->flags & DST_NOCACHE)) {
+        hold = (dst->flags & (DST_NOCACHE | DST_RCU_FREE)) == DST_NOCACHE;
+        if (unlikely(hold)) {
                 dst_hold(dst);
                 skb_dst_set(skb, dst);
         } else {
diff --git a/net/decnet/dn_route.c b/net/decnet/dn_route.c
index 85a3604c87c8..26719779ad8e 100644
--- a/net/decnet/dn_route.c
+++ b/net/decnet/dn_route.c
@@ -184,6 +184,12 @@ static __inline__ unsigned int dn_hash(__le16 src, __le16 dst)
         return dn_rt_hash_mask & (unsigned int)tmp;
 }
 
+static inline void dst_rcu_free(struct rcu_head *head)
+{
+        struct dst_entry *dst = container_of(head, struct dst_entry, rcu_head);
+        dst_free(dst);
+}
+
 static inline void dnrt_free(struct dn_route *rt)
 {
         call_rcu_bh(&rt->dst.rcu_head, dst_rcu_free);
diff --git a/net/ipv4/fib_semantics.c b/net/ipv4/fib_semantics.c
index da0cc2e6b250..e55171f184f9 100644
--- a/net/ipv4/fib_semantics.c
+++ b/net/ipv4/fib_semantics.c
@@ -172,9 +172,9 @@ static void free_fib_info_rcu(struct rcu_head *head)
                 if (nexthop_nh->nh_exceptions)
                         free_nh_exceptions(nexthop_nh);
                 if (nexthop_nh->nh_rth_output)
-                        dst_free(&nexthop_nh->nh_rth_output->dst);
+                        dst_release(&nexthop_nh->nh_rth_output->dst);
                 if (nexthop_nh->nh_rth_input)
-                        dst_free(&nexthop_nh->nh_rth_input->dst);
+                        dst_release(&nexthop_nh->nh_rth_input->dst);
         } endfor_nexthops(fi);
 
         release_net(fi->fib_net);
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index fc1a81ca79a7..d6eabcfe8a90 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1199,11 +1199,6 @@ restart:
         fnhe->fnhe_stamp = jiffies;
 }
 
-static inline void rt_free(struct rtable *rt)
-{
-        call_rcu_bh(&rt->dst.rcu_head, dst_rcu_free);
-}
-
 static void rt_cache_route(struct fib_nh *nh, struct rtable *rt)
 {
         struct rtable *orig, *prev, **p = &nh->nh_rth_output;
@@ -1213,17 +1208,14 @@ static void rt_cache_route(struct fib_nh *nh, struct rtable *rt)
 
         orig = *p;
 
+        rt->dst.flags |= DST_RCU_FREE;
+        dst_hold(&rt->dst);
         prev = cmpxchg(p, orig, rt);
         if (prev == orig) {
                 if (orig)
-                        rt_free(orig);
+                        dst_release(&orig->dst);
         } else {
-                /* Routes we intend to cache in the FIB nexthop have
-                 * the DST_NOCACHE bit clear.  However, if we are
-                 * unsuccessful at storing this route into the cache
-                 * we really need to set it.
-                 */
-                rt->dst.flags |= DST_NOCACHE;
+                dst_release(&rt->dst);
         }
 }
 
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a356e1fecf9a..9be30b039ae3 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5604,8 +5604,7 @@ void tcp_finish_connect(struct sock *sk, struct sk_buff *skb)
         tcp_set_state(sk, TCP_ESTABLISHED);
 
         if (skb != NULL) {
-                sk->sk_rx_dst = dst_clone(skb_dst(skb));
-                inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
+                inet_sk_rx_dst_set(sk, skb);
                 security_inet_conn_established(sk, skb);
         }
 
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 2fbd9921253f..7f91e5ac8277 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1617,19 +1617,19 @@ int tcp_v4_do_rcv(struct sock *sk, struct sk_buff *skb)
 #endif
 
         if (sk->sk_state == TCP_ESTABLISHED) { /* Fast path */
+                struct dst_entry *dst = sk->sk_rx_dst;
+
                 sock_rps_save_rxhash(sk, skb);
-                if (sk->sk_rx_dst) {
-                        struct dst_entry *dst = sk->sk_rx_dst;
+                if (dst) {
                         if (inet_sk(sk)->rx_dst_ifindex != skb->skb_iif ||
                             dst->ops->check(dst, 0) == NULL) {
                                 dst_release(dst);
                                 sk->sk_rx_dst = NULL;
                         }
                 }
-                if (unlikely(sk->sk_rx_dst == NULL)) {
-                        sk->sk_rx_dst = dst_clone(skb_dst(skb));
-                        inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
-                }
+                if (unlikely(sk->sk_rx_dst == NULL))
+                        inet_sk_rx_dst_set(sk, skb);
+
                 if (tcp_rcv_established(sk, skb, tcp_hdr(skb), skb->len)) {
                         rsk = sk;
                         goto reset;
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 3f1cc2028edd..232a90c3ec86 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -387,8 +387,7 @@ struct sock *tcp_create_openreq_child(struct sock *sk, struct request_sock *req,
                 struct tcp_sock *oldtp = tcp_sk(sk);
                 struct tcp_cookie_values *oldcvp = oldtp->cookie_values;
 
-                newsk->sk_rx_dst = dst_clone(skb_dst(skb));
-                inet_sk(newsk)->rx_dst_ifindex = skb->skb_iif;
+                inet_sk_rx_dst_set(newsk, skb);
 
                 /* TCP Cookie Transactions require space for the cookie pair,
                  * as it differs for each connection.  There is no need to