net_sched: avoid calling tcf_unbind_filter() in call_rcu callback

This fixes the following crash:

[   63.976822] general protection fault: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC
[   63.980094] CPU: 1 PID: 15 Comm: ksoftirqd/1 Not tainted 3.17.0-rc6+ #648
[   63.980094] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2011
[   63.980094] task: ffff880117dea690 ti: ffff880117dfc000 task.ti: ffff880117dfc000
[   63.980094] RIP: 0010:[<ffffffff817e6d07>]  [<ffffffff817e6d07>] u32_destroy_key+0x27/0x6d
[   63.980094] RSP: 0018:ffff880117dffcc0  EFLAGS: 00010202
[   63.980094] RAX: ffff880117dea690 RBX: ffff8800d02e0820 RCX: 0000000000000000
[   63.980094] RDX: 0000000000000001 RSI: 0000000000000002 RDI: 6b6b6b6b6b6b6b6b
[   63.980094] RBP: ffff880117dffcd0 R08: 0000000000000000 R09: 0000000000000000
[   63.980094] R10: 00006c0900006ba8 R11: 00006ba100006b9d R12: 0000000000000001
[   63.980094] R13: ffff8800d02e0898 R14: ffffffff817e6d4d R15: ffff880117387a30
[   63.980094] FS:  0000000000000000(0000) GS:ffff88011a800000(0000) knlGS:0000000000000000
[   63.980094] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[   63.980094] CR2: 00007f07e6732fed CR3: 000000011665b000 CR4: 00000000000006e0
[   63.980094] Stack:
[   63.980094]  ffff88011a9cd300 ffffffff82051ac0 ffff880117dffce0 ffffffff817e6d68
[   63.980094]  ffff880117dffd70 ffffffff810cb4c7 ffffffff810cb3cd ffff880117dfffd8
[   63.980094]  ffff880117dea690 ffff880117dea690 ffff880117dfffd8 000000000000000a
[   63.980094] Call Trace:
[   63.980094]  [<ffffffff817e6d68>] u32_delete_key_freepf_rcu+0x1b/0x1d
[   63.980094]  [<ffffffff810cb4c7>] rcu_process_callbacks+0x3bb/0x691
[   63.980094]  [<ffffffff810cb3cd>] ? rcu_process_callbacks+0x2c1/0x691
[   63.980094]  [<ffffffff817e6d4d>] ? u32_destroy_key+0x6d/0x6d
[   63.980094]  [<ffffffff810780a4>] __do_softirq+0x142/0x323
[   63.980094]  [<ffffffff810782a8>] run_ksoftirqd+0x23/0x53
[   63.980094]  [<ffffffff81092126>] smpboot_thread_fn+0x203/0x221
[   63.980094]  [<ffffffff81091f23>] ? smpboot_unpark_thread+0x33/0x33
[   63.980094]  [<ffffffff8108e44d>] kthread+0xc9/0xd1
[   63.980094]  [<ffffffff819e00ea>] ? do_wait_for_common+0xf8/0x125
[   63.980094]  [<ffffffff8108e384>] ? __kthread_parkme+0x61/0x61
[   63.980094]  [<ffffffff819e43ec>] ret_from_fork+0x7c/0xb0
[   63.980094]  [<ffffffff8108e384>] ? __kthread_parkme+0x61/0x61

tp could be freed in call_rcu callback too, the order is not guaranteed.

John Fastabend says:

====================
Its worth noting why this is safe. Any running schedulers will either
read the valid class field or it will be zeroed.

All schedulers today when the class is 0 do a lookup using the
same call used by the tcf_exts_bind(). So even if we have a running
classifier hit the null class pointer it will do a lookup and get
to the same result. This is particularly fragile at the moment because
the only way to verify this is to audit the schedulers call sites.
====================

Cc: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Acked-by: John Fastabend <john.r.fastabend@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/pkt_cls.h

@@ -20,11 +20,7 @@ int unregister_tcf_proto_ops(struct tcf_proto_ops *ops);
 static inline unsigned long
 __cls_set_class(unsigned long *clp, unsigned long cl)
 {
-	unsigned long old_cl;
- 
-	old_cl = *clp;
-	*clp = cl;
-	return old_cl;
+	return xchg(clp, cl);
 }
 
 static inline unsigned long

net/sched/cls_u32.c

@@ -358,7 +358,6 @@ static int u32_destroy_key(struct tcf_proto *tp,
 			   struct tc_u_knode *n,
 			   bool free_pf)
 {
-	tcf_unbind_filter(tp, &n->res);
 	tcf_exts_destroy(&n->exts);
 	if (n->ht_down)
 		n->ht_down->refcnt--;
@@ -416,6 +415,7 @@ static int u32_delete_key(struct tcf_proto *tp, struct tc_u_knode *key)
 			if (pkp == key) {
 				RCU_INIT_POINTER(*kp, key->next);
 
+				tcf_unbind_filter(tp, &key->res);
 				call_rcu(&key->rcu, u32_delete_key_freepf_rcu);
 				return 0;
 			}
@@ -425,7 +425,7 @@ static int u32_delete_key(struct tcf_proto *tp, struct tc_u_knode *key)
 	return 0;
 }
 
-static void u32_clear_hnode(struct tc_u_hnode *ht)
+static void u32_clear_hnode(struct tcf_proto *tp, struct tc_u_hnode *ht)
 {
 	struct tc_u_knode *n;
 	unsigned int h;
@@ -434,6 +434,7 @@ static void u32_clear_hnode(struct tc_u_hnode *ht)
 		while ((n = rtnl_dereference(ht->ht[h])) != NULL) {
 			RCU_INIT_POINTER(ht->ht[h],
 					 rtnl_dereference(n->next));
+			tcf_unbind_filter(tp, &n->res);
 			call_rcu(&n->rcu, u32_delete_key_freepf_rcu);
 		}
 	}
@@ -447,7 +448,7 @@ static int u32_destroy_hnode(struct tcf_proto *tp, struct tc_u_hnode *ht)
 
 	WARN_ON(ht->refcnt);
 
-	u32_clear_hnode(ht);
+	u32_clear_hnode(tp, ht);
 
 	hn = &tp_c->hlist;
 	for (phn = rtnl_dereference(*hn);
@@ -482,7 +483,7 @@ static void u32_destroy(struct tcf_proto *tp)
 		     ht;
 		     ht = rtnl_dereference(ht->next)) {
 			ht->refcnt--;
-			u32_clear_hnode(ht);
+			u32_clear_hnode(tp, ht);
 		}
 
 		while ((ht = rtnl_dereference(tp_c->hlist)) != NULL) {
@@ -731,6 +732,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		}
 
 		u32_replace_knode(tp, tp_c, new);
+		tcf_unbind_filter(tp, &n->res);
 		call_rcu(&n->rcu, u32_delete_key_rcu);
 		return 0;
 	}