[v4 PATCH 1/1] netfilter: Add fail-open support

[Krishna Kumar <krkumar2@in.ibm.com>]

Implement a new "fail-open" mode where packets are not dropped
upon queue-full condition. This mode can be enabled/disabled per
queue using netlink NFAQ_CFG_FLAGS & NFAQ_CFG_MASK attributes.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Signed-off-by: Vivek Kashyap <vivk@us.ibm.com>
Signed-off-by: Sridhar Samudrala <samudrala@us.ibm.com>
---
 include/linux/netfilter/nfnetlink_queue.h |    5 ++
 net/netfilter/core.c                      |   37 +++++++++++++++++++-
 net/netfilter/nf_queue.c                  |   15 ++++++--
 net/netfilter/nfnetlink_queue.c           |   36 +++++++++++++++++--
 4 files changed, 86 insertions(+), 7 deletions(-)

diff -ruNp org/include/linux/netfilter/nfnetlink_queue.h new/include/linux/netfilter/nfnetlink_queue.h
--- org/include/linux/netfilter/nfnetlink_queue.h	2012-05-23 09:52:54.738660685 +0530
+++ new/include/linux/netfilter/nfnetlink_queue.h	2012-05-24 10:25:33.500073415 +0530
@@ -84,8 +84,13 @@ enum nfqnl_attr_config {
 	NFQA_CFG_CMD,			/* nfqnl_msg_config_cmd */
 	NFQA_CFG_PARAMS,		/* nfqnl_msg_config_params */
 	NFQA_CFG_QUEUE_MAXLEN,		/* __u32 */
+	NFQA_CFG_MASK,			/* identify which flags to change */
+	NFQA_CFG_FLAGS,			/* value of these flags (__be32) */
 	__NFQA_CFG_MAX
 };
 #define NFQA_CFG_MAX (__NFQA_CFG_MAX-1)
 
+/* Flags for NFQA_CFG_FLAGS */
+#define NFQA_CFG_F_FAIL_OPEN			(1 << 0)
+
 #endif /* _NFNETLINK_QUEUE_H */
diff -ruNp org/net/netfilter/core.c new/net/netfilter/core.c
--- org/net/netfilter/core.c	2012-05-23 09:52:54.740660556 +0530
+++ new/net/netfilter/core.c	2012-05-24 11:35:55.958845493 +0530
@@ -163,6 +163,31 @@ repeat:
 	return NF_ACCEPT;
 }
 
+/*
+ * Handler was not able to enqueue the packet, and returned ENOSPC
+ * as "fail-open" was enabled. We temporarily accept the skb; or
+ * each segment for a GSO skb and free the header.
+ */
+static void handle_fail_open(struct sk_buff *skb,
+			     int (*okfn)(struct sk_buff *))
+{
+	struct sk_buff *segs;
+	bool gso;
+
+	segs = skb->next ? : skb;
+	gso = skb->next != NULL;
+
+	do {
+		struct sk_buff *nskb = segs->next;
+
+		segs->next = NULL;
+		okfn(segs);
+		segs = nskb;
+	} while (segs);
+
+	if (gso)
+		kfree_skb(skb);
+}
 
 /* Returns 1 if okfn() needs to be executed by the caller,
  * -EPERM for NF_DROP, 0 otherwise. */
@@ -174,6 +199,7 @@ int nf_hook_slow(u_int8_t pf, unsigned i
 {
 	struct list_head *elem;
 	unsigned int verdict;
+	int failopen = 0;
 	int ret = 0;
 
 	/* We may already have this, but read-locks nest anyway */
@@ -184,7 +210,8 @@ next_hook:
 	verdict = nf_iterate(&nf_hooks[pf][hook], skb, hook, indev,
 			     outdev, &elem, okfn, hook_thresh);
 	if (verdict == NF_ACCEPT || verdict == NF_STOP) {
-		ret = 1;
+		if (!failopen) /* don't use the default verdict if 'failopen' */
+			ret = 1;
 	} else if ((verdict & NF_VERDICT_MASK) == NF_DROP) {
 		kfree_skb(skb);
 		ret = NF_DROP_GETERR(verdict);
@@ -199,10 +226,18 @@ next_hook:
 			if (err == -ESRCH &&
 			   (verdict & NF_VERDICT_FLAG_QUEUE_BYPASS))
 				goto next_hook;
+			if (err == -ENOSPC) {
+				failopen = 1;
+				goto next_hook;
+			}
 			kfree_skb(skb);
 		}
 	}
 	rcu_read_unlock();
+
+	if (!ret && failopen)
+		handle_fail_open(skb, okfn);
+
 	return ret;
 }
 EXPORT_SYMBOL(nf_hook_slow);
diff -ruNp org/net/netfilter/nfnetlink_queue.c new/net/netfilter/nfnetlink_queue.c
--- org/net/netfilter/nfnetlink_queue.c	2012-05-23 09:52:54.742661899 +0530
+++ new/net/netfilter/nfnetlink_queue.c	2012-05-24 13:42:24.155860334 +0530
@@ -52,6 +52,7 @@ struct nfqnl_instance {
 
 	u_int16_t queue_num;			/* number of this queue */
 	u_int8_t copy_mode;
+	u_int32_t flags;			/* Set using NFQA_CFG_FLAGS */
 /*
  * Following fields are dirtied for each queued packet,
  * keep them in same cache line if possible.
@@ -431,9 +432,13 @@ nfqnl_enqueue_packet(struct nf_queue_ent
 		goto err_out_free_nskb;
 	}
 	if (queue->queue_total >= queue->queue_maxlen) {
-		queue->queue_dropped++;
-		net_warn_ratelimited("nf_queue: full at %d entries, dropping packets(s)\n",
-				     queue->queue_total);
+		if (queue->flags & NFQA_CFG_F_FAIL_OPEN) {
+			err = -ENOSPC;
+		} else {
+			queue->queue_dropped++;
+			net_warn_ratelimited("nf_queue: full at %d entries, dropping packets(s)\n",
+					     queue->queue_total);
+		}
 		goto err_out_free_nskb;
 	}
 	entry->id = ++queue->id_sequence;
@@ -858,6 +863,31 @@ nfqnl_recv_config(struct sock *ctnl, str
 		spin_unlock_bh(&queue->lock);
 	}
 
+	if (nfqa[NFQA_CFG_FLAGS]) {
+		__be32 flags, mask;
+
+		if (!queue) {
+			ret = -ENODEV;
+			goto err_out_unlock;
+		}
+
+		if (!nfqa[NFQA_CFG_MASK]) {
+			/* A mask is needed to specify which flags are being
+			 * changed.
+			 */
+			ret = -EINVAL;
+			goto err_out_unlock;
+		}
+
+		flags = ntohl(nla_get_be32(nfqa[NFQA_CFG_FLAGS]));
+		mask = ntohl(nla_get_be32(nfqa[NFQA_CFG_MASK]));
+
+		spin_lock_bh(&queue->lock);
+		queue->flags &= ~mask;
+		queue->flags |= flags & mask;
+		spin_unlock_bh(&queue->lock);
+	}
+
 err_out_unlock:
 	rcu_read_unlock();
 	return ret;
diff -ruNp org/net/netfilter/nf_queue.c new/net/netfilter/nf_queue.c
--- org/net/netfilter/nf_queue.c	2012-05-23 09:52:54.739533744 +0530
+++ new/net/netfilter/nf_queue.c	2012-05-24 11:34:46.302003629 +0530
@@ -268,14 +268,23 @@ int nf_queue(struct sk_buff *skb,
 			err = __nf_queue(segs, elem, pf, hook, indev,
 					   outdev, okfn, queuenum);
 		}
-		if (err == 0)
+
+		if (err == 0) {
 			queued++;
-		else
+		} else if (err == -ENOSPC) {
+			/* Enqueue failed due to queue-full and handler is
+			 * in "fail-open" mode.
+			 */
+			segs->next = nskb;
+			skb->next = segs;
+			break;
+		} else {
 			kfree_skb(segs);
+		}
 		segs = nskb;
 	} while (segs);
 
-	if (queued) {
+	if (queued && err != -ENOSPC) {
 		kfree_skb(skb);
 		return 0;
 	}