Patch "IB/ipoib: Fix race condition in neigh creation" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

IB/ipoib: Fix race condition in neigh creation

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ib-ipoib-fix-race-condition-in-neigh-creation.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Erez Shitrit <[email protected]>
Date: Sun, 31 Dec 2017 15:33:15 +0200
Subject: IB/ipoib: Fix race condition in neigh creation

From: Erez Shitrit <[email protected]>

[ Upstream commit 16ba3defb8bd01a9464ba4820a487f5b196b455b ]

When using enhanced mode for IPoIB, two threads may execute xmit in parallel to two different TX queues while the target is the same. In this case, both of them will add the same neighbor to the path's neigh link list and we might see the following message:

list_add double add: new=ffff88024767a348, prev=ffff88024767a348...

  WARNING: lib/list_debug.c:31__list_add_valid+0x4e/0x70
  ipoib_start_xmit+0x477/0x680 [ib_ipoib]
  dev_hard_start_xmit+0xb9/0x3e0
  sch_direct_xmit+0xf9/0x250
  __qdisc_run+0x176/0x5d0
  __dev_queue_xmit+0x1f5/0xb10
  __dev_queue_xmit+0x55/0xb10

Analysis:
Two SKB are scheduled to be transmitted from two cores.
In ipoib_start_xmit, both gets NULL when calling ipoib_neigh_get. Two calls to neigh_add_path are made. One thread takes the spin-lock and calls ipoib_neigh_alloc which creates the neigh structure, then (after the __path_find) the neigh is added to the path's neigh link list. When the second thread enters the critical section it also calls ipoib_neigh_alloc but in this case it gets the already allocated ipoib_neigh structure, which is already linked to the path's neigh link list and adds it again to the list. Which beside of triggering the list, it creates a loop in the linked list. This loop leads to endless loop inside path_rec_completion.

Solution:
Check list_empty(&neigh->list) before adding to the list. Add a similar fix in "ipoib_multicast.c::ipoib_mcast_send"

---
 drivers/infiniband/ulp/ipoib/ipoib_main.c      |   25 ++++++++++++++++++-------
 drivers/infiniband/ulp/ipoib/ipoib_multicast.c |    5 ++++-
 2 files changed, 22 insertions(+), 8 deletions(-)

@@ -844,8 +844,8 @@ static int path_rec_start(struct net_dev
 	return 0;
 }
 
-static void neigh_add_path(struct sk_buff *skb, u8 *daddr,
-			   struct net_device *dev)
+static struct ipoib_neigh *neigh_add_path(struct sk_buff *skb, u8 *daddr,
+					  struct net_device *dev)
 {
 	struct ipoib_dev_priv *priv = netdev_priv(dev);
 	struct ipoib_path *path;

@@ -858,7 +858,15 @@ static void neigh_add_path(struct sk_buf
 		spin_unlock_irqrestore(&priv->lock, flags);
 		++dev->stats.tx_dropped;
 		dev_kfree_skb_any(skb);
-		return;
+		return NULL;
+	}
+
+	/* To avoid race condition, make sure that the
+	 * neigh will be added only once.
+	 */
+	if (unlikely(!list_empty(&neigh->list))) {
+		spin_unlock_irqrestore(&priv->lock, flags);
+		return neigh;
 	}
 
 	path = __path_find(dev, daddr + 4);

@@ -896,7 +904,7 @@ static void neigh_add_path(struct sk_buf
 			spin_unlock_irqrestore(&priv->lock, flags);
 			ipoib_send(dev, skb, path->ah, IPOIB_QPN(daddr));
 			ipoib_neigh_put(neigh);
-			return;
+			return NULL;
 		}
 	} else {
 		neigh->ah  = NULL;

@@ -913,7 +921,7 @@ static void neigh_add_path(struct sk_buf
 
 	spin_unlock_irqrestore(&priv->lock, flags);
 	ipoib_neigh_put(neigh);
-	return;
+	return NULL;
 
 err_path:
 	ipoib_neigh_free(neigh);

@@ -923,6 +931,8 @@ err_drop:
 
 	spin_unlock_irqrestore(&priv->lock, flags);
 	ipoib_neigh_put(neigh);
+
+	return NULL;
 }
 
 static void unicast_arp_send(struct sk_buff *skb, struct net_device *dev,

@@ -1028,8 +1038,9 @@ static int ipoib_start_xmit(struct sk_bu
 	case htons(ETH_P_TIPC):
 		neigh = ipoib_neigh_get(dev, phdr->hwaddr);
 		if (unlikely(!neigh)) {
-			neigh_add_path(skb, phdr->hwaddr, dev);
-			return NETDEV_TX_OK;
+			neigh = neigh_add_path(skb, phdr->hwaddr, dev);
+			if (likely(!neigh))
+				return NETDEV_TX_OK;
 		}
 		break;
 	case htons(ETH_P_ARP):

@@ -775,7 +775,10 @@ void ipoib_mcast_send(struct net_device
 		spin_lock_irqsave(&priv->lock, flags);
 		if (!neigh) {
 			neigh = ipoib_neigh_alloc(daddr, dev);
-			if (neigh) {
+			/* Make sure that the neigh will be added only
+			 * once to mcast list.
+			 */
+			if (neigh && list_empty(&neigh->list)) {
 				kref_get(&mcast->ah->ref);
 				neigh->ah	= mcast->ah;
 				list_add_tail(&neigh->list, &mcast->neigh_list);

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

queue-4.4/ib-ipoib-fix-race-condition-in-neigh-creation.patch