Patch "neigh: Explicitly declare RCU-bh read side critical section in neigh_xmit()" has been added to the 4.6-stable tree

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

neigh: Explicitly declare RCU-bh read side critical section in neigh_xmit()

to the 4.6-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:
neigh-explicitly-declare-rcu-bh-read-side-critical-section-in-neigh_xmit.patch
and it can be found in the queue-4.6 subdirectory.

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

From: David Barroso <[email protected]>
Date: Tue, 28 Jun 2016 11:16:43 +0300
Subject: neigh: Explicitly declare RCU-bh read side critical section in neigh_xmit()

From: David Barroso <[email protected]>

[ Upstream commit b560f03ddfb072bca65e9440ff0dc4f9b1d1f056 ]

neigh_xmit() expects to be called inside an RCU-bh read side critical section, and while one of its two current callers gets this right, the other one doesn't.

More specifically, neigh_xmit() has two callers, mpls_forward() and mpls_output(), and while both callers call neigh_xmit() under rcu_read_lock(), this provides sufficient protection for neigh_xmit() only in the case of mpls_forward(), as that is always called from softirq context and therefore doesn't need explicit BH protection, while mpls_output() can be called from process context with softirqs enabled.

When mpls_output() is called from process context, with softirqs enabled, we can be preempted by a softirq at any time, and RCU-bh considers the completion of a softirq as signaling the end of any pending read-side critical sections, so if we do get a softirq while we are in the part of neigh_xmit() that expects to be run inside an RCU-bh read side critical section, we can end up with an unexpected RCU grace period running right in the middle of that critical section, making things go boom.

This patch fixes this impedance mismatch in the callee, by making neigh_xmit() always take rcu_read_{,un}lock_bh() around the code that expects to be treated as an RCU-bh read side critical section, as this seems a safer option than fixing it in the callers.

---
 net/core/neighbour.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

@@ -2467,13 +2467,17 @@ int neigh_xmit(int index, struct net_dev
 		tbl = neigh_tables[index];
 		if (!tbl)
 			goto out;
+		rcu_read_lock_bh();
 		neigh = __neigh_lookup_noref(tbl, addr, dev);
 		if (!neigh)
 			neigh = __neigh_create(tbl, addr, dev, false);
 		err = PTR_ERR(neigh);
-		if (IS_ERR(neigh))
+		if (IS_ERR(neigh)) {
+			rcu_read_unlock_bh();
 			goto out_kfree_skb;
+		}
 		err = neigh->output(neigh, skb);
+		rcu_read_unlock_bh();
 	}
 	else if (index == NEIGH_LINK_TABLE) {
 		err = dev_hard_header(skb, dev, ntohs(skb->protocol),

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

queue-4.6/neigh-explicitly-declare-rcu-bh-read-side-critical-section-in-neigh_xmit.patch