<quote David>
I am seeing a crash on a distro V4.2.3 kernel caused by a double release of a dst_entry. In ipv4_dst_destroy() the call to list_empty() finds a poisoned next pointer, indicating the dst_entry has already been removed from the list and freed. The crash occurs 18 to 24 hours into a run of a network stress exerciser.
</quote>

Thanks to his detailed report and analysis, we were able to understand the core issue.

IP early demux can associate a dst to skb, after a lookup in TCP/UDP sockets.

When socket cache is not properly set, we want to store into sk->sk_dst_cache the dst for future IP early demux lookups, by acquiring a stable refcount on the dst.

Problem is this acquisition is simply using an atomic_inc(), which works well, unless the dst was queued for destruction from dst_release() noticing dst refcount went to zero, if DST_NOCACHE was set on dst.

We need to make sure current refcount is not zero before incrementing it, or risk double free as David reported.

This patch, being a stable candidate, adds two new helpers, and use them only from IP early demux problematic paths.

It might be possible to merge in net-next skb_dst_force() and skb_dst_force_safe(), but I prefer having the smallest patch for stable kernels : Maybe some skb_dst_force() callers do not expect skb->dst can suddenly be cleared.

Can probably be backported back to linux-3.6 kernels

Reported-by David J. Wilder <[email protected]>

Tested-by David J. Wilder <[email protected]>

Signed-off-by Eric Dumazet <[email protected]>

Signed-off-by David S. Miller <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 include/net/dst.h   |   33 +++++++++++++++++++++++++++++++++
 include/net/sock.h  |    2 +-
 net/ipv4/tcp_ipv4.c |    5 ++---
 net/ipv6/tcp_ipv6.c |    3 +--
 4 files changed, 37 insertions(+), 6 deletions(-)

include/net/dst.h +33 -0

@@ -322,6 +322,39 @@ static inline void skb_dst_force(struct
 	}
 }
 
+/**
+ * dst_hold_safe - Take a reference on a dst if possible
+ * @dst: pointer to dst entry
+ *
+ * This helper returns false if it could not safely
+ * take a reference on a dst.
+ */
+static inline bool dst_hold_safe(struct dst_entry *dst)
+{
+	if (dst->flags & DST_NOCACHE)
+		return atomic_inc_not_zero(&dst->__refcnt);
+	dst_hold(dst);
+	return true;
+}
+
+/**
+ * skb_dst_force_safe - makes sure skb dst is refcounted
+ * @skb: buffer
+ *
+ * If dst is not yet refcounted and not destroyed, grab a ref on it.
+ */
+static inline void skb_dst_force_safe(struct sk_buff *skb)
+{
+	if (skb_dst_is_noref(skb)) {
+		struct dst_entry *dst = skb_dst(skb);
+
+		if (!dst_hold_safe(dst))
+			dst = NULL;
+
+		skb->_skb_refdst = (unsigned long)dst;
+	}
+}
+
 
 /**
  *	__skb_tunnel_rx - prepare skb for rx reinsert

include/net/sock.h +1 -1

@@ -801,7 +801,7 @@ void sk_stream_write_space(struct sock *
 static inline void __sk_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
 	/* dont let skb dst not refcounted, we are going to leave rcu lock */
-	skb_dst_force(skb);
+	skb_dst_force_safe(skb);
 
 	if (!sk->sk_backlog.tail)
 		sk->sk_backlog.head = skb;

net/ipv4/tcp_ipv4.c +2 -3

@@ -1509,7 +1509,7 @@ bool tcp_prequeue(struct sock *sk, struc
 	if (likely(sk->sk_rx_dst))
 		skb_dst_drop(skb);
 	else
-		skb_dst_force(skb);
+		skb_dst_force_safe(skb);
 
 	__skb_queue_tail(&tp->ucopy.prequeue, skb);
 	tp->ucopy.memory += skb->truesize;

@@ -1710,8 +1710,7 @@ void inet_sk_rx_dst_set(struct sock *sk,
 {
 	struct dst_entry *dst = skb_dst(skb);
 
-	if (dst) {
-		dst_hold(dst);
+	if (dst && dst_hold_safe(dst)) {
 		sk->sk_rx_dst = dst;
 		inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
 	}

net/ipv6/tcp_ipv6.c +1 -2

@@ -93,10 +93,9 @@ static void inet6_sk_rx_dst_set(struct s
 {
 	struct dst_entry *dst = skb_dst(skb);
 
-	if (dst) {
+	if (dst && dst_hold_safe(dst)) {
 		const struct rt6_info *rt = (const struct rt6_info *)dst;
 
-		dst_hold(dst);
 		sk->sk_rx_dst = dst;
 		inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
 		inet6_sk(sk)->rx_dst_cookie = rt6_get_cookie(rt);

Patches currently in stable-queue which might be from [email protected] are

queue-4.3/inet-tcp-fix-inetpeer_set_addr_v4.patch queue-4.3/ipv6-sctp-clone-options-to-avoid-use-after-free.patch queue-4.3/tcp-restore-fastopen-with-no-data-in-syn-packet.patch queue-4.3/net_sched-make-qdisc_tree_decrease_qlen-work-for-non-mq.patch queue-4.3/net-fix-ip-early-demux-races.patch