counter: 104-quad-8: Fix race getting function mode and direction

The quad8_action_read() function checks the Count function mode and
Count direction without first acquiring a lock. This is a race condition
because the function mode could change by the time the direction is
checked.

Because the quad8_function_read() already acquires a lock internally,
the quad8_function_read() is refactored to spin out the no-lock code to
a new quad8_function_get() function.

To resolve the race condition in quad8_action_read(), a lock is acquired
before calling quad8_function_get() and quad8_direction_read() in order
to get both function mode and direction atomically.

Fixes: f1d8a07 ("counter: 104-quad-8: Add Generic Counter interface support")
Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]/
Signed-off-by: William Breathitt Gray <[email protected]>

Author: William Breathitt Gray

drivers/counter/104-quad-8.c

@@ -232,34 +232,45 @@ static const enum counter_function quad8_count_functions_list[] = {
 	COUNTER_FUNCTION_QUADRATURE_X4,
 };
 
+static int quad8_function_get(const struct quad8 *const priv, const size_t id,
+			      enum counter_function *const function)
+{
+	if (!priv->quadrature_mode[id]) {
+		*function = COUNTER_FUNCTION_PULSE_DIRECTION;
+		return 0;
+	}
+
+	switch (priv->quadrature_scale[id]) {
+	case 0:
+		*function = COUNTER_FUNCTION_QUADRATURE_X1_A;
+		return 0;
+	case 1:
+		*function = COUNTER_FUNCTION_QUADRATURE_X2_A;
+		return 0;
+	case 2:
+		*function = COUNTER_FUNCTION_QUADRATURE_X4;
+		return 0;
+	default:
+		/* should never reach this path */
+		return -EINVAL;
+	}
+}
+
 static int quad8_function_read(struct counter_device *counter,
 			       struct counter_count *count,
 			       enum counter_function *function)
 {
 	struct quad8 *const priv = counter_priv(counter);
-	const int id = count->id;
 	unsigned long irqflags;
+	int retval;
 
 	spin_lock_irqsave(&priv->lock, irqflags);
 
-	if (priv->quadrature_mode[id])
-		switch (priv->quadrature_scale[id]) {
-		case 0:
-			*function = COUNTER_FUNCTION_QUADRATURE_X1_A;
-			break;
-		case 1:
-			*function = COUNTER_FUNCTION_QUADRATURE_X2_A;
-			break;
-		case 2:
-			*function = COUNTER_FUNCTION_QUADRATURE_X4;
-			break;
-		}
-	else
-		*function = COUNTER_FUNCTION_PULSE_DIRECTION;
+	retval = quad8_function_get(priv, count->id, function);
 
 	spin_unlock_irqrestore(&priv->lock, irqflags);
 
-	return 0;
+	return retval;
 }
 
 static int quad8_function_write(struct counter_device *counter,
@@ -359,6 +370,7 @@ static int quad8_action_read(struct counter_device *counter,
 			     enum counter_synapse_action *action)
 {
 	struct quad8 *const priv = counter_priv(counter);
+	unsigned long irqflags;
 	int err;
 	enum counter_function function;
 	const size_t signal_a_id = count->synapses[0].signal->id;
@@ -374,9 +386,21 @@ static int quad8_action_read(struct counter_device *counter,
 		return 0;
 	}
 
-	err = quad8_function_read(counter, count, &function);
-	if (err)
+	spin_lock_irqsave(&priv->lock, irqflags);
+
+	/* Get Count function and direction atomically */
+	err = quad8_function_get(priv, count->id, &function);
+	if (err) {
+		spin_unlock_irqrestore(&priv->lock, irqflags);
+		return err;
+	}
+	err = quad8_direction_read(counter, count, &direction);
+	if (err) {
+		spin_unlock_irqrestore(&priv->lock, irqflags);
 		return err;
+	}
+
+	spin_unlock_irqrestore(&priv->lock, irqflags);
 
 	/* Default action mode */
 	*action = COUNTER_SYNAPSE_ACTION_NONE;
@@ -389,10 +413,6 @@ static int quad8_action_read(struct counter_device *counter,
 		return 0;
 	case COUNTER_FUNCTION_QUADRATURE_X1_A:
 		if (synapse->signal->id == signal_a_id) {
-			err = quad8_direction_read(counter, count, &direction);
-			if (err)
-				return err;
-
 			if (direction == COUNTER_COUNT_DIRECTION_FORWARD)
 				*action = COUNTER_SYNAPSE_ACTION_RISING_EDGE;
 			else