tomoyo: Use atomic_t for statistics counter

syzbot is reporting that there is a race at tomoyo_stat_update() [1].
Although it is acceptable to fail to track exact number of times policy
was updated, convert to atomic_t because this is not a hot path.

[1] https://syzkaller.appspot.com/bug?id=a4d7b973972eeed410596e6604580e0133b0fc04

Reported-by: syzbot <[email protected]>
Signed-off-by: Tetsuo Handa <[email protected]>

security/tomoyo/common.c

@@ -2322,9 +2322,9 @@ static const char * const tomoyo_memory_headers[TOMOYO_MAX_MEMORY_STAT] = {
 	[TOMOYO_MEMORY_QUERY]  = "query message:",
 };
 
-/* Timestamp counter for last updated. */
-static unsigned int tomoyo_stat_updated[TOMOYO_MAX_POLICY_STAT];
 /* Counter for number of updates. */
+static atomic_t tomoyo_stat_updated[TOMOYO_MAX_POLICY_STAT];
+/* Timestamp counter for last updated. */
 static time64_t tomoyo_stat_modified[TOMOYO_MAX_POLICY_STAT];
 
 /**
@@ -2336,10 +2336,7 @@ static time64_t tomoyo_stat_modified[TOMOYO_MAX_POLICY_STAT];
  */
 void tomoyo_update_stat(const u8 index)
 {
-	/*
-	 * I don't use atomic operations because race condition is not fatal.
-	 */
-	tomoyo_stat_updated[index]++;
+	atomic_inc(&tomoyo_stat_updated[index]);
 	tomoyo_stat_modified[index] = ktime_get_real_seconds();
 }
 
@@ -2360,7 +2357,7 @@ static void tomoyo_read_stat(struct tomoyo_io_buffer *head)
 	for (i = 0; i < TOMOYO_MAX_POLICY_STAT; i++) {
 		tomoyo_io_printf(head, "Policy %-30s %10u",
 				 tomoyo_policy_headers[i],
-				 tomoyo_stat_updated[i]);
+				 atomic_read(&tomoyo_stat_updated[i]));
 		if (tomoyo_stat_modified[i]) {
 			struct tomoyo_time stamp;