apparmor: document the buffer hold, add an overflow guard

The buffer hold is a measure of contention, but it is tracked per cpu
where the lock is a globabl resource. On some systems (eg. real time)
there is no guarantee that the code will be on the same cpu pre, and
post spinlock acquisition, nor that the buffer will be put back to
the same percpu cache when we are done with it.

Because of this the hold value can move asynchronous to the buffers on
the cache, meaning it is possible to underflow, and potentially in really
pathelogical cases overflow.

Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

Author: jrjohansen

security/apparmor/lsm.c

@@ -2125,6 +2125,23 @@ static int param_set_mode(const char *val, const struct kernel_param *kp)
 	return 0;
 }
 
+/* arbitrary cap on how long to hold buffer because contention was
+ * encountered before trying to put it back into the global pool
+ */
+#define MAX_HOLD_COUNT 64
+
+/* the hold count is a heuristic for lock contention, and can be
+ * incremented async to actual buffer alloc/free.  Because buffers
+ * may be put back onto a percpu cache different than the ->hold was
+ * added to the counts can be out of sync. Guard against underflow
+ * and overflow
+ */
+static void cache_hold_inc(unsigned int *hold)
+{
+	if (*hold > MAX_HOLD_COUNT)
+		(*hold)++;
+}
+
 char *aa_get_buffer(bool in_atomic)
 {
 	union aa_buffer *aa_buf;
@@ -2143,11 +2160,18 @@ char *aa_get_buffer(bool in_atomic)
 		put_cpu_ptr(&aa_local_buffers);
 		return &aa_buf->buffer[0];
 	}
+	/* exit percpu as spinlocks may sleep on realtime kernels */
 	put_cpu_ptr(&aa_local_buffers);
 
 	if (!spin_trylock(&aa_buffers_lock)) {
+		/* had contention on lock so increase hold count. Doesn't
+		 * really matter if recorded before or after the spin lock
+		 * as there is no way to guarantee the buffer will be put
+		 * back on the same percpu cache. Instead rely on holds
+		 * roughly averaging out over time.
+		 */
 		cache = get_cpu_ptr(&aa_local_buffers);
-		cache->hold += 1;
+		cache_hold_inc(&cache->hold);
 		put_cpu_ptr(&aa_local_buffers);
 		spin_lock(&aa_buffers_lock);
 	} else {
@@ -2213,7 +2237,7 @@ void aa_put_buffer(char *buf)
 		}
 		/* contention on global list, fallback to percpu */
 		cache = get_cpu_ptr(&aa_local_buffers);
-		cache->hold += 1;
+		cache_hold_inc(&cache->hold);
 	}
 
 	/* cache in percpu list */