staging: android: ashmem: fix a race condition in ASHMEM_SET_SIZE ioctl

A lock-unlock is missing in ASHMEM_SET_SIZE ioctl which can result in a
race condition when mmap is called. After the !asma->file check, before
setting asma->size, asma->file can be set in mmap. That would result in
having different asma->size than the mapped memory size. Combined with
ASHMEM_UNPIN ioctl and shrinker invocation, this can result in memory
corruption.

Bug: 66954097
Signed-off-by: Viktor Slavkovic <viktors@google.com>
Change-Id: Ia52312a75ade30bc94be6b94420f17f34e0c1f86

drivers/staging/android/ashmem.c

@@ -672,10 +672,12 @@ static long ashmem_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		break;
 	case ASHMEM_SET_SIZE:
 		ret = -EINVAL;
+		mutex_lock(&ashmem_mutex);
 		if (!asma->file) {
 			ret = 0;
 			asma->size = (size_t) arg;
 		}
+		mutex_unlock(&ashmem_mutex);
 		break;
 	case ASHMEM_GET_SIZE:
 		ret = asma->size;