[PATCH batadv] batman-adv: dat: atomically update mac addresses

When a MAC address is updated in batadv_dat_entry_add(), it is done using a
simple copy function. A parallel reader might only see parts of this
update. In worst case, the reader is transporting the half updated MAC
address over the network or is creating an ARP response using it -
poisoning the ARP cache.

atomic64_t can be used to store the 48 bit of a mac address. A reader will
then either see the old mac address or the new one - never a mixture of
both.

Reported-by: Sashiko <[email protected]>
Fixes: f6badf9eb582 ("batman-adv: Distributed ARP Table - implement local storage")
Signed-off-by: Sven Eckelmann <[email protected]>
---
 net/batman-adv/distributed-arp-table.c | 58 +++++++++++++++++++++++-----------
 net/batman-adv/types.h                 |  2 +-
 2 files changed, 40 insertions(+), 20 deletions(-)

diff --git a/net/batman-adv/distributed-arp-table.c b/net/batman-adv/distributed-arp-table.c
index a5c9b6d0..6b16ebd7 100644
--- a/net/batman-adv/distributed-arp-table.c
+++ b/net/batman-adv/distributed-arp-table.c
@@ -440,18 +440,19 @@ batadv_dat_entry_hash_find(struct batadv_priv *bat_priv, __be32 ip,
 static void batadv_dat_entry_add(struct batadv_priv *bat_priv, __be32 ip,
 				 u8 *mac_addr, unsigned short vid)
 {
+	u64 u64_mac = ether_addr_to_u64(mac_addr);
 	struct batadv_dat_entry *dat_entry;
 	int hash_added;
 
 	dat_entry = batadv_dat_entry_hash_find(bat_priv, ip, vid);
 	/* if this entry is already known, just update it */
 	if (dat_entry) {
-		if (!batadv_compare_eth(dat_entry->mac_addr, mac_addr))
-			ether_addr_copy(dat_entry->mac_addr, mac_addr);
+		atomic64_set(&dat_entry->mac_addr, u64_mac);
+
 		dat_entry->last_update = jiffies;
 		batadv_dbg(BATADV_DBG_DAT, bat_priv,
 			   "Entry updated: %pI4 %pM (vid: %d)\n",
-			   &dat_entry->ip, dat_entry->mac_addr,
+			   &dat_entry->ip, mac_addr,
 			   batadv_print_vid(vid));
 		goto out;
 	}
@@ -462,7 +463,7 @@ static void batadv_dat_entry_add(struct batadv_priv *bat_priv, __be32 ip,
 
 	dat_entry->ip = ip;
 	dat_entry->vid = vid;
-	ether_addr_copy(dat_entry->mac_addr, mac_addr);
+	atomic64_set(&dat_entry->mac_addr, u64_mac);
 	dat_entry->last_update = jiffies;
 	kref_init(&dat_entry->refcount);
 
@@ -478,7 +479,7 @@ static void batadv_dat_entry_add(struct batadv_priv *bat_priv, __be32 ip,
 	}
 
 	batadv_dbg(BATADV_DBG_DAT, bat_priv, "New entry added: %pI4 %pM (vid: %d)\n",
-		   &dat_entry->ip, dat_entry->mac_addr, batadv_print_vid(vid));
+		   &dat_entry->ip, mac_addr, batadv_print_vid(vid));
 
 out:
 	batadv_dat_entry_put(dat_entry);
@@ -935,6 +936,8 @@ batadv_dat_cache_dump_entry(struct sk_buff *msg, u32 portid,
 			    struct netlink_callback *cb,
 			    struct batadv_dat_entry *dat_entry)
 {
+	u8 mac[ETH_ALEN];
+	u64 u64_mac;
 	int msecs;
 	void *hdr;
 
@@ -947,11 +950,12 @@ batadv_dat_cache_dump_entry(struct sk_buff *msg, u32 portid,
 	genl_dump_check_consistent(cb, hdr);
 
 	msecs = jiffies_to_msecs(jiffies - dat_entry->last_update);
+	u64_mac = atomic64_read(&dat_entry->mac_addr);
+	u64_to_ether_addr(u64_mac, mac);
 
 	if (nla_put_in_addr(msg, BATADV_ATTR_DAT_CACHE_IP4ADDRESS,
 			    dat_entry->ip) ||
-	    nla_put(msg, BATADV_ATTR_DAT_CACHE_HWADDRESS, ETH_ALEN,
-		    dat_entry->mac_addr) ||
+	    nla_put(msg, BATADV_ATTR_DAT_CACHE_HWADDRESS, ETH_ALEN, mac) ||
 	    nla_put_u16(msg, BATADV_ATTR_DAT_CACHE_VID, dat_entry->vid) ||
 	    nla_put_u32(msg, BATADV_ATTR_LAST_SEEN_MSECS, msecs)) {
 		genlmsg_cancel(msg, hdr);
@@ -1231,10 +1235,12 @@ bool batadv_dat_snoop_outgoing_arp_request(struct batadv_priv *bat_priv,
 	struct sk_buff *skb_new;
 	unsigned short vid;
 	bool ret = false;
+	u8 mac[ETH_ALEN];
 	int hdr_size = 0;
 	__be32 ip_dst;
 	__be32 ip_src;
 	u16 type = 0;
+	u64 u64_mac;
 	u8 *hw_src;
 
 	if (!READ_ONCE(bat_priv->distributed_arp_table))
@@ -1263,6 +1269,9 @@ bool batadv_dat_snoop_outgoing_arp_request(struct batadv_priv *bat_priv,
 
 	dat_entry = batadv_dat_entry_hash_find(bat_priv, ip_dst, vid);
 	if (dat_entry) {
+		u64_mac = atomic64_read(&dat_entry->mac_addr);
+		u64_to_ether_addr(u64_mac, mac);
+
 		/* If the ARP request is destined for a local client the local
 		 * client will answer itself. DAT would only generate a
 		 * duplicate packet.
@@ -1271,7 +1280,7 @@ bool batadv_dat_snoop_outgoing_arp_request(struct batadv_priv *bat_priv,
 		 * additional DAT answer may trigger kernel warnings about
 		 * a packet coming from the wrong port.
 		 */
-		if (batadv_is_my_client(bat_priv, dat_entry->mac_addr, vid)) {
+		if (batadv_is_my_client(bat_priv, mac, vid)) {
 			ret = true;
 			goto out;
 		}
@@ -1281,18 +1290,16 @@ bool batadv_dat_snoop_outgoing_arp_request(struct batadv_priv *bat_priv,
 		 * the backbone gws belonging to our backbone has claimed the
 		 * destination.
 		 */
-		if (!batadv_bla_check_claim(bat_priv,
-					    dat_entry->mac_addr, vid)) {
+		if (!batadv_bla_check_claim(bat_priv, mac, vid)) {
 			batadv_dbg(BATADV_DBG_DAT, bat_priv,
 				   "Device %pM claimed by another backbone gw. Don't send ARP reply!",
-				   dat_entry->mac_addr);
+				   mac);
 			ret = true;
 			goto out;
 		}
 
 		skb_new = batadv_dat_arp_create_reply(bat_priv, ip_dst, ip_src,
-						      dat_entry->mac_addr,
-						      hw_src, vid);
+						      mac, hw_src, vid);
 		if (!skb_new)
 			goto out;
 
@@ -1336,8 +1343,10 @@ bool batadv_dat_snoop_incoming_arp_request(struct batadv_priv *bat_priv,
 	struct sk_buff *skb_new;
 	unsigned short vid;
 	bool ret = false;
+	u8 mac[ETH_ALEN];
 	__be32 ip_src;
 	__be32 ip_dst;
+	u64 u64_mac;
 	u8 *hw_src;
 	u16 type;
 	int err;
@@ -1367,8 +1376,11 @@ bool batadv_dat_snoop_incoming_arp_request(struct batadv_priv *bat_priv,
 	if (!dat_entry)
 		goto out;
 
+	u64_mac = atomic64_read(&dat_entry->mac_addr);
+	u64_to_ether_addr(u64_mac, mac);
+
 	skb_new = batadv_dat_arp_create_reply(bat_priv, ip_dst, ip_src,
-					      dat_entry->mac_addr, hw_src, vid);
+					      mac, hw_src, vid);
 	if (!skb_new)
 		goto out;
 
@@ -1460,8 +1472,10 @@ bool batadv_dat_snoop_incoming_arp_reply(struct batadv_priv *bat_priv,
 	struct batadv_dat_entry *dat_entry = NULL;
 	bool dropped = false;
 	unsigned short vid;
+	u8 mac[ETH_ALEN];
 	__be32 ip_src;
 	__be32 ip_dst;
+	u64 u64_mac;
 	u8 *hw_src;
 	u8 *hw_dst;
 	u16 type;
@@ -1493,11 +1507,17 @@ bool batadv_dat_snoop_incoming_arp_reply(struct batadv_priv *bat_priv,
 	 * this frame would lead to doubled receive of an ARP reply.
 	 */
 	dat_entry = batadv_dat_entry_hash_find(bat_priv, ip_src, vid);
-	if (dat_entry && batadv_compare_eth(hw_src, dat_entry->mac_addr)) {
-		batadv_dbg(BATADV_DBG_DAT, bat_priv, "Doubled ARP reply removed: ARP MSG = [src: %pM-%pI4 dst: %pM-%pI4]; dat_entry: %pM-%pI4\n",
-			   hw_src, &ip_src, hw_dst, &ip_dst,
-			   dat_entry->mac_addr,	&dat_entry->ip);
-		dropped = true;
+	if (dat_entry) {
+		u64_mac = atomic64_read(&dat_entry->mac_addr);
+		u64_to_ether_addr(u64_mac, mac);
+
+		if (batadv_compare_eth(hw_src, mac)) {
+			batadv_dbg(BATADV_DBG_DAT, bat_priv,
+				   "Doubled ARP reply removed: ARP MSG = [src: %pM-%pI4 dst: %pM-%pI4]; dat_entry: %pM-%pI4\n",
+				   hw_src, &ip_src, hw_dst, &ip_dst,
+				   mac, &dat_entry->ip);
+			dropped = true;
+		}
 	}
 
 	/* Update our internal cache with both the IP addresses the node got
diff --git a/net/batman-adv/types.h b/net/batman-adv/types.h
index 688cf233..d6e15d46 100644
--- a/net/batman-adv/types.h
+++ b/net/batman-adv/types.h
@@ -2176,7 +2176,7 @@ struct batadv_dat_entry {
 	__be32 ip;
 
 	/** @mac_addr: the MAC address associated to the stored IPv4 */
-	u8 mac_addr[ETH_ALEN];
+	atomic64_t mac_addr;
 
 	/** @vid: the vlan ID associated to this entry */
 	unsigned short vid;

---
base-commit: e281f6726677f7e8950dad1640d5386867e6d2e6
change-id: 20260706-dat-atomic-mac-0c580ee47619

Best regards,
--  
Sven Eckelmann <[email protected]>