Patch "net, sched: respect rcu grace period on cls destruction" 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

net, sched: respect rcu grace period on cls destruction

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:
net-sched-respect-rcu-grace-period-on-cls-destruction.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: Daniel Borkmann <[email protected]>
Date: Sun, 27 Nov 2016 01:18:01 +0100
Subject: net, sched: respect rcu grace period on cls destruction

From: Daniel Borkmann <[email protected]>

[ Upstream commit d936377414fadbafb4d17148d222fe45ca5442d4 ]

Roi reported a crash in flower where tp->root was NULL in ->classify() callbacks. Reason is that in ->destroy() tp->root is set to NULL via RCU_INIT_POINTER(). It's problematic for some of the classifiers, because this doesn't respect RCU grace period for them, and as a result, still outstanding readers from tc_classify() will try to blindly dereference a NULL tp->root.

The tp->root object is strictly private to the classifier implementation and holds internal data the core such as tc_ctl_tfilter() doesn't know about. Within some classifiers, such as cls_bpf, cls_basic, etc, tp->root is only checked for NULL in ->get() callback, but nowhere else. This is misleading and seemed to be copied from old classifier code that was not cleaned up properly. For example, d3fa76ee6b4a ("[NET_SCHED]: cls_basic:
fix NULL pointer dereference") moved tp->root initialization into ->init() routine, where before it was part of ->change(), so ->get() had to deal with tp->root being NULL back then, so that was indeed a valid case, after d3fa76ee6b4a, not really anymore. We used to set tp->root to NULL long ago in ->destroy(), see 47a1a1d4be29 ("pkt_sched: remove unnecessary xchg() in packet classifiers"); but the NULLifying was reintroduced with the RCUification, but it's not correct for every classifier implementation.

In the cases that are fixed here with one exception of cls_cgroup, tp->root object is allocated and initialized inside ->init() callback, which is always performed at a point in time after we allocate a new tp, which means tp and thus tp->root was not globally visible in the tp chain yet (see tc_ctl_tfilter()). Also, on destruction tp->root is strictly kfree_rcu()'ed in ->destroy() handler, same for the tp which is kfree_rcu()'ed right when we return from ->destroy() in tcf_destroy(). This means, the head object's lifetime for such classifiers is always tied to the tp lifetime. The RCU callback invocation for the two kfree_rcu() could be out of order, but that's fine since both are independent.

Dropping the RCU_INIT_POINTER(tp->root, NULL) for these classifiers here means that 1) we don't need a useless NULL check in fast-path and, 2) that outstanding readers of that tp in tc_classify() can still execute under respect with RCU grace period as it is actually expected.

Things that haven't been touched here: cls_fw and cls_route. They each handle tp->root being NULL in ->classify() path for historic reasons, so their ->destroy() implementation can stay as is. If someone actually cares, they could get cleaned up at some point to avoid the test in fast path. cls_u32 doesn't set tp->root to NULL. For cls_rsvp, I just added a !head should anyone actually be using/testing it, so it at least aligns with cls_fw and cls_route. For cls_flower we additionally need to defer rhashtable destruction (to a sleepable context) after RCU grace period as concurrent readers might still access it. (Note that in this case we need to hold module reference to keep work callback address intact, since we only wait on module unload for all call_rcu()s to finish.)

This fixes one race to bring RCU grace period guarantees back. Next step as worked on by Cong however is to fix 1e052be69d04 ("net_sched: destroy proto tp when all filters are gone") to get the order of unlinking the tp in tc_ctl_tfilter() for the RTM_DELTFILTER case right by moving RCU_INIT_POINTER() before tcf_destroy() and let the notification for removal be done through the prior ->delete() callback. Both are independant issues. Once we have that right, we can then clean tp->root up for a number of classifiers by not making them RCU pointers, which requires a new callback (->uninit) that is triggered from tp's RCU callback, where we just kfree() tp->root from there.

---
 net/sched/cls_basic.c   |    4 ----
 net/sched/cls_bpf.c     |    4 ----
 net/sched/cls_cgroup.c  |    7 +++----
 net/sched/cls_flow.c    |    1 -
 net/sched/cls_flower.c  |   31 ++++++++++++++++++++++++++-----
 net/sched/cls_rsvp.h    |    3 ++-
 net/sched/cls_tcindex.c |    1 -
 7 files changed, 31 insertions(+), 20 deletions(-)

@@ -62,9 +62,6 @@ static unsigned long basic_get(struct tc
 	struct basic_head *head = rtnl_dereference(tp->root);
 	struct basic_filter *f;
 
-	if (head == NULL)
-		return 0UL;
-
 	list_for_each_entry(f, &head->flist, link) {
 		if (f->handle == handle) {
 			l = (unsigned long) f;

@@ -109,7 +106,6 @@ static bool basic_destroy(struct tcf_pro
 		tcf_unbind_filter(tp, &f->res);
 		call_rcu(&f->rcu, basic_delete_filter);
 	}
-	RCU_INIT_POINTER(tp->root, NULL);
 	kfree_rcu(head, rcu);
 	return true;
 }

@@ -199,7 +199,6 @@ static bool cls_bpf_destroy(struct tcf_p
 		call_rcu(&prog->rcu, __cls_bpf_delete_prog);
 	}
 
-	RCU_INIT_POINTER(tp->root, NULL);
 	kfree_rcu(head, rcu);
 	return true;
 }

@@ -210,9 +209,6 @@ static unsigned long cls_bpf_get(struct
 	struct cls_bpf_prog *prog;
 	unsigned long ret = 0UL;
 
-	if (head == NULL)
-		return 0UL;
-
 	list_for_each_entry(prog, &head->plist, link) {
 		if (prog->handle == handle) {
 			ret = (unsigned long) prog;

@@ -130,11 +130,10 @@ static bool cls_cgroup_destroy(struct tc
 
 	if (!force)
 		return false;
-
-	if (head) {
-		RCU_INIT_POINTER(tp->root, NULL);
+	/* Head can still be NULL due to cls_cgroup_init(). */
+	if (head)
 		call_rcu(&head->rcu, cls_cgroup_destroy_rcu);
-	}
+
 	return true;
 }
 

@@ -583,7 +583,6 @@ static bool flow_destroy(struct tcf_prot
 		list_del_rcu(&f->list);
 		call_rcu(&f->rcu, flow_destroy_filter);
 	}
-	RCU_INIT_POINTER(tp->root, NULL);
 	kfree_rcu(head, rcu);
 	return true;
 }

@@ -13,6 +13,7 @@
 #include <linux/init.h>
 #include <linux/module.h>
 #include <linux/rhashtable.h>
+#include <linux/workqueue.h>
 
 #include <linux/if_ether.h>
 #include <linux/in6.h>

@@ -55,7 +56,10 @@ struct cls_fl_head {
 	bool mask_assigned;
 	struct list_head filters;
 	struct rhashtable_params ht_params;
-	struct rcu_head rcu;
+	union {
+		struct work_struct work;
+		struct rcu_head	rcu;
+	};
 };
 
 struct cls_fl_filter {

@@ -165,6 +169,24 @@ static void fl_destroy_filter(struct rcu
 	kfree(f);
 }
 
+static void fl_destroy_sleepable(struct work_struct *work)
+{
+	struct cls_fl_head *head = container_of(work, struct cls_fl_head,
+						work);
+	if (head->mask_assigned)
+		rhashtable_destroy(&head->ht);
+	kfree(head);
+	module_put(THIS_MODULE);
+}
+
+static void fl_destroy_rcu(struct rcu_head *rcu)
+{
+	struct cls_fl_head *head = container_of(rcu, struct cls_fl_head, rcu);
+
+	INIT_WORK(&head->work, fl_destroy_sleepable);
+	schedule_work(&head->work);
+}
+
 static bool fl_destroy(struct tcf_proto *tp, bool force)
 {
 	struct cls_fl_head *head = rtnl_dereference(tp->root);

@@ -177,10 +199,9 @@ static bool fl_destroy(struct tcf_proto
 		list_del_rcu(&f->list);
 		call_rcu(&f->rcu, fl_destroy_filter);
 	}
-	RCU_INIT_POINTER(tp->root, NULL);
-	if (head->mask_assigned)
-		rhashtable_destroy(&head->ht);
-	kfree_rcu(head, rcu);
+
+	__module_get(THIS_MODULE);
+	call_rcu(&head->rcu, fl_destroy_rcu);
 	return true;
 }
 

@@ -152,7 +152,8 @@ static int rsvp_classify(struct sk_buff
 		return -1;
 	nhptr = ip_hdr(skb);
 #endif
-
+	if (unlikely(!head))
+		return -1;
 restart:
 
 #if RSVP_DST_LEN == 4

@@ -503,7 +503,6 @@ static bool tcindex_destroy(struct tcf_p
 	walker.fn = tcindex_destroy_element;
 	tcindex_walk(tp, &walker);
 
-	RCU_INIT_POINTER(tp->root, NULL);
 	call_rcu(&p->rcu, __tcindex_destroy);
 	return true;
 }

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

queue-4.4/net-sched-respect-rcu-grace-period-on-cls-destruction.patch