[PATCH net-next 06/15] batman-adv: dat: annotate last_update with READ/WRITE_ONCE

[Simon Wunderlich <sw@simonwunderlich.de>]

From: Sven Eckelmann <sven@narfation.org>

The last_update field for batadv_dat_entry tracks the jiffies value of
the most recent activity and is used to detect timeouts. These accesses are
not consistently protected by a lock, so READ_ONCE/WRITE_ONCE must be used
to prevent data races caused by compiler optimizations.

To avoid an underflow due to an non-strict order read of jiffies vs
last_update, use jiffies_delta_to_msecs instead of jiffies_to_msecs to
evaluate the delta.

Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/bug/linux-4933a0a0-367e-43b3-8da5-29ae37dfdff3
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
---
 net/batman-adv/distributed-arp-table.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/batman-adv/distributed-arp-table.c b/net/batman-adv/distributed-arp-table.c
index 0260ed7e828b2..7e82c154c13f8 100644
--- a/net/batman-adv/distributed-arp-table.c
+++ b/net/batman-adv/distributed-arp-table.c
@@ -11,6 +11,7 @@
 #include <linux/bitops.h>
 #include <linux/bug.h>
 #include <linux/byteorder/generic.h>
+#include <linux/compiler.h>
 #include <linux/container_of.h>
 #include <linux/err.h>
 #include <linux/errno.h>
@@ -206,7 +207,7 @@ static void batadv_dat_entry_put(struct batadv_dat_entry *dat_entry)
  */
 static bool batadv_dat_to_purge(struct batadv_dat_entry *dat_entry)
 {
-	return batadv_has_timed_out(dat_entry->last_update,
+	return batadv_has_timed_out(READ_ONCE(dat_entry->last_update),
 				    BATADV_DAT_ENTRY_TIMEOUT);
 }
 
@@ -453,7 +454,7 @@ static void batadv_dat_entry_add(struct batadv_priv *bat_priv, __be32 ip,
 	if (dat_entry) {
 		atomic64_set(&dat_entry->mac_addr, u64_mac);
 
-		dat_entry->last_update = jiffies;
+		WRITE_ONCE(dat_entry->last_update, jiffies);
 		batadv_dbg(BATADV_DBG_DAT, bat_priv,
 			   "Entry updated: %pI4 %pM (vid: %d)\n",
 			   &dat_entry->ip, mac_addr,
@@ -468,7 +469,7 @@ static void batadv_dat_entry_add(struct batadv_priv *bat_priv, __be32 ip,
 	dat_entry->ip = ip;
 	dat_entry->vid = vid;
 	atomic64_set(&dat_entry->mac_addr, u64_mac);
-	dat_entry->last_update = jiffies;
+	WRITE_ONCE(dat_entry->last_update, jiffies);
 	kref_init(&dat_entry->refcount);
 
 	kref_get(&dat_entry->refcount);
@@ -940,6 +941,7 @@ batadv_dat_cache_dump_entry(struct sk_buff *msg, u32 portid,
 			    struct batadv_dat_entry *dat_entry)
 {
 	u8 mac[ETH_ALEN] __aligned(2);
+	unsigned long last_update;
 	u64 u64_mac;
 	int msecs;
 	void *hdr;
@@ -952,7 +954,8 @@ 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);
+	last_update = READ_ONCE(dat_entry->last_update);
+	msecs = jiffies_delta_to_msecs(jiffies - last_update);
 	u64_mac = atomic64_read(&dat_entry->mac_addr);
 	u64_to_ether_addr(u64_mac, mac);
 
-- 
2.47.3