net: add a noref bit on skb dst

Use low order bit of skb->_skb_dst to tell dst is not refcounted.

Change _skb_dst to _skb_refdst to make sure all uses are catched.

skb_dst() returns the dst, regardless of noref bit set or not, but
with a lockdep check to make sure a noref dst is not given if current
user is not rcu protected.

New skb_dst_set_noref() helper to set an notrefcounted dst on a skb.
(with lockdep check)

skb_dst_drop() drops a reference only if skb dst was refcounted.

skb_dst_force() helper is used to force a refcount on dst, when skb
is queued and not anymore RCU protected.

Use skb_dst_force() in __sk_add_backlog(), __dev_xmit_skb() if
!IFF_XMIT_DST_RELEASE or skb enqueued on qdisc queue, in
sock_queue_rcv_skb(), in __nf_queue().

Use skb_dst_force() in dev_requeue_skb().

Note: dst_use_noref() still dirties dst, we might transform it
later to do one dirtying per jiffies.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

12 files changed, 134 insertions, 25 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index c9525bce80f6..7cdfb4d52847 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -264,7 +264,7 @@ typedef unsigned char *sk_buff_data_t;
  *      @transport_header: Transport layer header
  *      @network_header: Network layer header
  *      @mac_header: Link layer header
- *      @_skb_dst: destination entry
+ *      @_skb_refdst: destination entry (with norefcount bit)
  *      @sp: the security path, used for xfrm
  *      @cb: Control buffer. Free for use by every layer. Put private vars here
  *      @len: Length of actual data
@@ -328,7 +328,7 @@ struct sk_buff {
          */
         char                    cb[48] __aligned(8);
 
-        unsigned long           _skb_dst;
+        unsigned long           _skb_refdst;
 #ifdef CONFIG_XFRM
         struct  sec_path        *sp;
 #endif
@@ -419,14 +419,64 @@ struct sk_buff {
 
 #include <asm/system.h>
 
+/*
+ * skb might have a dst pointer attached, refcounted or not.
+ * _skb_refdst low order bit is set if refcount was _not_ taken
+ */
+#define SKB_DST_NOREF   1UL
+#define SKB_DST_PTRMASK ~(SKB_DST_NOREF)
+
+/**
+ * skb_dst - returns skb dst_entry
+ * @skb: buffer
+ *
+ * Returns skb dst_entry, regardless of reference taken or not.
+ */
 static inline struct dst_entry *skb_dst(const struct sk_buff *skb)
 {
-        return (struct dst_entry *)skb->_skb_dst;
+        /* If refdst was not refcounted, check we still are in a 
+         * rcu_read_lock section
+         */
+        WARN_ON((skb->_skb_refdst & SKB_DST_NOREF) &&
+                !rcu_read_lock_held() &&
+                !rcu_read_lock_bh_held());
+        return (struct dst_entry *)(skb->_skb_refdst & SKB_DST_PTRMASK);
 }
 
+/**
+ * skb_dst_set - sets skb dst
+ * @skb: buffer
+ * @dst: dst entry
+ *
+ * Sets skb dst, assuming a reference was taken on dst and should
+ * be released by skb_dst_drop()
+ */
 static inline void skb_dst_set(struct sk_buff *skb, struct dst_entry *dst)
 {
-        skb->_skb_dst = (unsigned long)dst;
+        skb->_skb_refdst = (unsigned long)dst;
+}
+
+/**
+ * skb_dst_set_noref - sets skb dst, without a reference
+ * @skb: buffer
+ * @dst: dst entry
+ *
+ * Sets skb dst, assuming a reference was not taken on dst
+ * skb_dst_drop() should not dst_release() this dst
+ */
+static inline void skb_dst_set_noref(struct sk_buff *skb, struct dst_entry *dst)
+{
+        WARN_ON(!rcu_read_lock_held() && !rcu_read_lock_bh_held());
+        skb->_skb_refdst = (unsigned long)dst | SKB_DST_NOREF;
+}
+
+/**
+ * skb_dst_is_noref - Test if skb dst isnt refcounted
+ * @skb: buffer
+ */
+static inline bool skb_dst_is_noref(const struct sk_buff *skb)
+{
+        return (skb->_skb_refdst & SKB_DST_NOREF) && skb_dst(skb);
 }
 
 static inline struct rtable *skb_rtable(const struct sk_buff *skb)
diff --git a/include/net/dst.h b/include/net/dst.h
index aac5a5fcfda9..27207a13f2a6 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -168,6 +168,12 @@ static inline void dst_use(struct dst_entry *dst, unsigned long time)
         dst->lastuse = time;
 }
 
+static inline void dst_use_noref(struct dst_entry *dst, unsigned long time)
+{
+        dst->__use++;
+        dst->lastuse = time;
+}
+
 static inline
 struct dst_entry * dst_clone(struct dst_entry * dst)
 {
@@ -177,11 +183,47 @@ struct dst_entry * dst_clone(struct dst_entry * dst)
 }
 
 extern void dst_release(struct dst_entry *dst);
+
+static inline void refdst_drop(unsigned long refdst)
+{
+        if (!(refdst & SKB_DST_NOREF))
+                dst_release((struct dst_entry *)(refdst & SKB_DST_PTRMASK));
+}
+
+/**
+ * skb_dst_drop - drops skb dst
+ * @skb: buffer
+ *
+ * Drops dst reference count if a reference was taken.
+ */
 static inline void skb_dst_drop(struct sk_buff *skb)
 {
-        if (skb->_skb_dst)
-                dst_release(skb_dst(skb));
-        skb->_skb_dst = 0UL;
+        if (skb->_skb_refdst) {
+                refdst_drop(skb->_skb_refdst);
+                skb->_skb_refdst = 0UL;
+        }
+}
+
+static inline void skb_dst_copy(struct sk_buff *nskb, const struct sk_buff *oskb)
+{
+        nskb->_skb_refdst = oskb->_skb_refdst;
+        if (!(nskb->_skb_refdst & SKB_DST_NOREF))
+                dst_clone(skb_dst(nskb));
+}
+
+/**
+ * skb_dst_force - makes sure skb dst is refcounted
+ * @skb: buffer
+ *
+ * If dst is not yet refcounted, let's do it
+ */
+static inline void skb_dst_force(struct sk_buff *skb)
+{
+        if (skb_dst_is_noref(skb)) {
+                WARN_ON(!rcu_read_lock_held());
+                skb->_skb_refdst &= ~SKB_DST_NOREF;
+                dst_clone(skb_dst(skb));
+        }
 }
 
 /* Children define the path of the packet through the
diff --git a/include/net/sock.h b/include/net/sock.h
index aed16eb9db4b..5697caf8cc76 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -600,12 +600,15 @@ static inline int sk_stream_memory_free(struct sock *sk)
 /* OOB backlog add */
 static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
-        if (!sk->sk_backlog.tail) {
-                sk->sk_backlog.head = sk->sk_backlog.tail = skb;
-        } else {
+        /* dont let skb dst not refcounted, we are going to leave rcu lock */
+        skb_dst_force(skb);
+
+        if (!sk->sk_backlog.tail)
+                sk->sk_backlog.head = skb;
+        else
                 sk->sk_backlog.tail->next = skb;
-                sk->sk_backlog.tail = skb;
-        }
+
+        sk->sk_backlog.tail = skb;
         skb->next = NULL;
 }
 
diff --git a/net/core/dev.c b/net/core/dev.c
index cdcb9cbedf41..6c820650b80f 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -2052,6 +2052,8 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
                  * waiting to be sent out; and the qdisc is not running -
                  * xmit the skb directly.
                  */
+                if (!(dev->priv_flags & IFF_XMIT_DST_RELEASE))
+                        skb_dst_force(skb);
                 __qdisc_update_bstats(q, skb->len);
                 if (sch_direct_xmit(skb, q, dev, txq, root_lock))
                         __qdisc_run(q);
@@ -2060,6 +2062,7 @@ static inline int __dev_xmit_skb(struct sk_buff *skb, struct Qdisc *q,
 
                 rc = NET_XMIT_SUCCESS;
         } else {
+                skb_dst_force(skb);
                 rc = qdisc_enqueue_root(skb, q);
                 qdisc_run(q);
         }
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a9b0e1f77806..c543dd252433 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -520,7 +520,7 @@ static void __copy_skb_header(struct sk_buff *new, const struct sk_buff *old)
         new->transport_header   = old->transport_header;
         new->network_header     = old->network_header;
         new->mac_header         = old->mac_header;
-        skb_dst_set(new, dst_clone(skb_dst(old)));
+        skb_dst_copy(new, old);
         new->rxhash             = old->rxhash;
 #ifdef CONFIG_XFRM
         new->sp                 = secpath_get(old->sp);
diff --git a/net/core/sock.c b/net/core/sock.c
index 63530a03b8c2..bf88a167c8f2 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -307,6 +307,11 @@ int sock_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
          */
         skb_len = skb->len;
 
+        /* we escape from rcu protected region, make sure we dont leak
+         * a norefcounted dst
+         */
+        skb_dst_force(skb);
+
         spin_lock_irqsave(&list->lock, flags);
         skb->dropcount = atomic_read(&sk->sk_drops);
         __skb_queue_tail(list, skb);
@@ -1536,6 +1541,7 @@ static void __release_sock(struct sock *sk)
                 do {
                         struct sk_buff *next = skb->next;
 
+                        WARN_ON_ONCE(skb_dst_is_noref(skb));
                         skb->next = NULL;
                         sk_backlog_rcv(sk, skb);
 
diff --git a/net/ipv4/icmp.c b/net/ipv4/icmp.c
index f3d339f728b0..d65e9215bcd7 100644
--- a/net/ipv4/icmp.c
+++ b/net/ipv4/icmp.c
@@ -587,20 +587,20 @@ void icmp_send(struct sk_buff *skb_in, int type, int code, __be32 info)
                         err = __ip_route_output_key(net, &rt2, &fl);
                 else {
                         struct flowi fl2 = {};
-                        struct dst_entry *odst;
+                        unsigned long orefdst;
 
                         fl2.fl4_dst = fl.fl4_src;
                         if (ip_route_output_key(net, &rt2, &fl2))
                                 goto relookup_failed;
 
                         /* Ugh! */
-                        odst = skb_dst(skb_in);
+                        orefdst = skb_in->_skb_refdst; /* save old refdst */
                         err = ip_route_input(skb_in, fl.fl4_dst, fl.fl4_src,
                                              RT_TOS(tos), rt2->u.dst.dev);
 
                         dst_release(&rt2->u.dst);
                         rt2 = skb_rtable(skb_in);
-                        skb_dst_set(skb_in, odst);
+                        skb_in->_skb_refdst = orefdst; /* restore old refdst */
                 }
 
                 if (err)
diff --git a/net/ipv4/ip_options.c b/net/ipv4/ip_options.c
index 4c09a31fd140..3244133c24f6 100644
--- a/net/ipv4/ip_options.c
+++ b/net/ipv4/ip_options.c
@@ -601,6 +601,7 @@ int ip_options_rcv_srr(struct sk_buff *skb)
         unsigned char *optptr = skb_network_header(skb) + opt->srr;
         struct rtable *rt = skb_rtable(skb);
         struct rtable *rt2;
+        unsigned long orefdst;
         int err;
 
         if (!opt->srr)
@@ -624,16 +625,16 @@ int ip_options_rcv_srr(struct sk_buff *skb)
                 }
                 memcpy(&nexthop, &optptr[srrptr-1], 4);
 
-                rt = skb_rtable(skb);
+                orefdst = skb->_skb_refdst;
                 skb_dst_set(skb, NULL);
                 err = ip_route_input(skb, nexthop, iph->saddr, iph->tos, skb->dev);
                 rt2 = skb_rtable(skb);
                 if (err || (rt2->rt_type != RTN_UNICAST && rt2->rt_type != RTN_LOCAL)) {
-                        ip_rt_put(rt2);
-                        skb_dst_set(skb, &rt->u.dst);
+                        skb_dst_drop(skb);
+                        skb->_skb_refdst = orefdst;
                         return -EINVAL;
                 }
-                ip_rt_put(rt);
+                refdst_drop(orefdst);
                 if (rt2->rt_type != RTN_LOCAL)
                         break;
                 /* Superfast 8) loopback forward */
diff --git a/net/ipv4/netfilter.c b/net/ipv4/netfilter.c
index 82fb43c5c59e..07de855e2175 100644
--- a/net/ipv4/netfilter.c
+++ b/net/ipv4/netfilter.c
@@ -17,7 +17,7 @@ int ip_route_me_harder(struct sk_buff *skb, unsigned addr_type)
         const struct iphdr *iph = ip_hdr(skb);
         struct rtable *rt;
         struct flowi fl = {};
-        struct dst_entry *odst;
+        unsigned long orefdst;
         unsigned int hh_len;
         unsigned int type;
 
@@ -51,14 +51,14 @@ int ip_route_me_harder(struct sk_buff *skb, unsigned addr_type)
                 if (ip_route_output_key(net, &rt, &fl) != 0)
                         return -1;
 
-                odst = skb_dst(skb);
+                orefdst = skb->_skb_refdst;
                 if (ip_route_input(skb, iph->daddr, iph->saddr,
                                    RT_TOS(iph->tos), rt->u.dst.dev) != 0) {
                         dst_release(&rt->u.dst);
                         return -1;
                 }
                 dst_release(&rt->u.dst);
-                dst_release(odst);
+                refdst_drop(orefdst);
         }
 
         if (skb_dst(skb)->error)
diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index dea3f9264250..705eccfb4769 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -3033,7 +3033,7 @@ int ip_rt_dump(struct sk_buff *skb,  struct netlink_callback *cb)
                                 continue;
                         if (rt_is_expired(rt))
                                 continue;
-                        skb_dst_set(skb, dst_clone(&rt->u.dst));
+                        skb_dst_set_noref(skb, &rt->u.dst);
                         if (rt_fill_info(net, skb, NETLINK_CB(cb->skb).pid,
                                          cb->nlh->nlmsg_seq, RTM_NEWROUTE,
                                          1, NLM_F_MULTI) <= 0) {
diff --git a/net/netfilter/nf_queue.c b/net/netfilter/nf_queue.c
index 0b1103c0b1f3..78b3cf9c519c 100644
--- a/net/netfilter/nf_queue.c
+++ b/net/netfilter/nf_queue.c
@@ -9,6 +9,7 @@
 #include <linux/rcupdate.h>
 #include <net/protocol.h>
 #include <net/netfilter/nf_queue.h>
+#include <net/dst.h>
 
 #include "nf_internals.h"
 
@@ -170,6 +171,7 @@ static int __nf_queue(struct sk_buff *skb,
                         dev_hold(physoutdev);
         }
 #endif
+        skb_dst_force(skb);
         afinfo->saveroute(skb, entry);
         status = qh->outfn(entry, queuenum);
 
diff --git a/net/sched/sch_generic.c b/net/sched/sch_generic.c
index a969b111bd76..a63029ef3edd 100644
--- a/net/sched/sch_generic.c
+++ b/net/sched/sch_generic.c
@@ -26,6 +26,7 @@
 #include <linux/list.h>
 #include <linux/slab.h>
 #include <net/pkt_sched.h>
+#include <net/dst.h>
 
 /* Main transmission queue. */
 
@@ -40,6 +41,7 @@
 
 static inline int dev_requeue_skb(struct sk_buff *skb, struct Qdisc *q)
 {
+        skb_dst_force(skb);
         q->gso_skb = skb;
         q->qstats.requeues++;
         q->q.qlen++;    /* it's still part of the queue */
@@ -179,7 +181,7 @@ static inline int qdisc_restart(struct Qdisc *q)
         skb = dequeue_skb(q);
         if (unlikely(!skb))
                 return 0;
-
+        WARN_ON_ONCE(skb_dst_is_noref(skb));
         root_lock = qdisc_lock(q);
         dev = qdisc_dev(q);
         txq = netdev_get_tx_queue(dev, skb_get_queue_mapping(skb));