netdev - Re: Double free of dst_entry in ipv4_dst

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Message-ID: <1449852442.9768.21.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Fri, 11 Dec 2015 08:47:22 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	dwilder@...ibm.com
Cc:	netdev@...r.kernel.org, predeep@...ibm.com, mjtarsel@...ibm.com
Subject: Re: Double free of dst_entry in ipv4_dst_destroy()

On Fri, 2015-12-11 at 07:48 -0800, Eric Dumazet wrote:
> On Fri, 2015-12-11 at 06:23 -0800, Eric Dumazet wrote:
> > On Sun, 2015-12-06 at 17:58 -0800, Eric Dumazet wrote:
> > > On Sun, 2015-12-06 at 13:03 -0800, Eric Dumazet wrote:
> > > 
> > > > But then when later we promote a skb->dst to a refctounted one
> > > > (skb_dst_force(), we might make sure we abort the operation if __refcnt
> > > > == 0 ( and DST_NOCACHE is in dst->flags)
> > > > 
> > > 
> > > Minimum patch would be :
> > > 
> > 
> > Here is a more complete patch, it should fix the issue I think :
> 
> Hmm, I'll send a v3, I forgot to test DST_NOCACHE properly.
> 

David, please test the following patch, thanks !

 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(-)

diff --git a/include/net/dst.h b/include/net/dst.h
index 1279f9b09791..c7329dcd90cc 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -322,6 +322,39 @@ static inline void skb_dst_force(struct sk_buff *skb)
 	}
 }
 
+/**
+ * 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
diff --git a/include/net/sock.h b/include/net/sock.h
index eaef41433d7a..18322bded064 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -816,7 +816,7 @@ void sk_stream_write_space(struct sock *sk);
 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;
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index db003438aaf5..d8841a2f1569 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1493,7 +1493,7 @@ bool tcp_prequeue(struct sock *sk, struct sk_buff *skb)
 	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;
@@ -1721,8 +1721,7 @@ void inet_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
 {
 	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;
 	}
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index e7aab561b7b4..6b8a8a9091fa 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -93,10 +93,9 @@ static void inet6_sk_rx_dst_set(struct sock *sk, const struct sk_buff *skb)
 {
 	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);


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