Patch "arp: honour gratuitous ARP _replies_" has been added to the 4.9-stable tree

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

arp: honour gratuitous ARP _replies_

to the 4.9-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:
arp-honour-gratuitous-arp-_replies_.patch
and it can be found in the queue-4.9 subdirectory.

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

From: Ihar Hrachyshka <[email protected]>
Date: Tue, 16 May 2017 07:53:43 -0700
Subject: arp: honour gratuitous ARP _replies_

From: Ihar Hrachyshka <[email protected]>

[ Upstream commit 23d268eb240954e6e78f7cfab04f2b1e79f84489 ]

When arp_accept is 1, gratuitous ARPs are supposed to override matching entries irrespective of whether they arrive during locktime. This was implemented in commit 56022a8fdd87 ("ipv4: arp: update neighbour address when a gratuitous arp is received and arp_accept is set")

There is a glitch in the patch though. RFC 2002, section 4.6, "ARP, Proxy ARP, and Gratuitous ARP", defines gratuitous ARPs so that they can be either of Request or Reply type. Those Reply gratuitous ARPs can be triggered with standard tooling, for example, arping -A option does just that.

This patch fixes the glitch, making both Request and Reply flavours of gratuitous ARPs to behave identically.

As per RFC, if gratuitous ARPs are of Reply type, their Target Hardware Address field should also be set to the link-layer address to which this cache entry should be updated. The field is present in ARP over Ethernet but not in IEEE 1394. In this patch, I don't consider any broadcasted ARP replies as gratuitous if the field is not present, to conform the standard. It's not clear whether there is such a thing for IEEE 1394 as a gratuitous ARP reply; until it's cleared up, we will ignore such broadcasts. Note that they will still update existing ARP cache entries, assuming they arrive out of locktime time interval.

---
 net/ipv4/arp.c |   16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

@@ -658,6 +658,7 @@ static int arp_process(struct net *net,
 	unsigned char *arp_ptr;
 	struct rtable *rt;
 	unsigned char *sha;
+	unsigned char *tha = NULL;
 	__be32 sip, tip;
 	u16 dev_type = dev->type;
 	int addr_type;

@@ -729,6 +730,7 @@ static int arp_process(struct net *net,
 		break;
 #endif
 	default:
+		tha = arp_ptr;
 		arp_ptr += dev->addr_len;
 	}
 	memcpy(&tip, arp_ptr, 4);

@@ -847,8 +849,18 @@ static int arp_process(struct net *net,
 		   It is possible, that this option should be enabled for some
 		   devices (strip is candidate)
 		 */
-		is_garp = arp->ar_op == htons(ARPOP_REQUEST) && tip == sip &&
-			  addr_type == RTN_UNICAST;
+		is_garp = tip == sip && addr_type == RTN_UNICAST;
+
+		/* Unsolicited ARP _replies_ also require target hwaddr to be
+		 * the same as source.
+		 */
+		if (is_garp && arp->ar_op == htons(ARPOP_REPLY))
+			is_garp =
+				/* IPv4 over IEEE 1394 doesn't provide target
+				 * hardware address field in its ARP payload.
+				 */
+				tha &&
+				!memcmp(tha, sha, dev->addr_len);
 
 		if (!n &&
 		    ((arp->ar_op == htons(ARPOP_REPLY)  &&

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

queue-4.9/arp-honour-gratuitous-arp-_replies_.patch queue-4.9/neighbour-update-neigh-timestamps-iff-update-is-effective.patch