staging: android: ashmem: Fix a race condition in pin ioctls

commit ce8a3a9e upstream.

ashmem_pin_unpin() reads asma->file and asma->size before taking the
ashmem_mutex, so it can race with other operations that modify them.

Build-tested only.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/ashmem.c

@@ -704,30 +704,32 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
 	size_t pgstart, pgend;
 	int ret = -EINVAL;
 
+	mutex_lock(&ashmem_mutex);
+
 	if (unlikely(!asma->file))
-		return -EINVAL;
+		goto out_unlock;
 
-	if (unlikely(copy_from_user(&pin, p, sizeof(pin))))
-		return -EFAULT;
+	if (unlikely(copy_from_user(&pin, p, sizeof(pin)))) {
+		ret = -EFAULT;
+		goto out_unlock;
+	}
 
 	/* per custom, you can pass zero for len to mean "everything onward" */
 	if (!pin.len)
 		pin.len = PAGE_ALIGN(asma->size) - pin.offset;
 
 	if (unlikely((pin.offset | pin.len) & ~PAGE_MASK))
-		return -EINVAL;
+		goto out_unlock;
 
 	if (unlikely(((__u32)-1) - pin.offset < pin.len))
-		return -EINVAL;
+		goto out_unlock;
 
 	if (unlikely(PAGE_ALIGN(asma->size) < pin.offset + pin.len))
-		return -EINVAL;
+		goto out_unlock;
 
 	pgstart = pin.offset / PAGE_SIZE;
 	pgend = pgstart + (pin.len / PAGE_SIZE) - 1;
 
-	mutex_lock(&ashmem_mutex);
-
 	switch (cmd) {
 	case ASHMEM_PIN:
 		ret = ashmem_pin(asma, pgstart, pgend);
@@ -740,6 +742,7 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
 		break;
 	}
 
+out_unlock:
 	mutex_unlock(&ashmem_mutex);
 
 	return ret;