Use user mount with matching shared storage only

Signed-off-by: Julius Härtl <jus@bitgrid.net>

lib/private/Share20/Manager.php

@@ -43,6 +43,7 @@
 use OC\Files\Mount\MoveableMount;
 use OC\HintException;
 use OC\Share20\Exception\ProviderException;
+use OCA\Files_Sharing\ISharedStorage;
 use OCP\EventDispatcher\IEventDispatcher;
 use OCP\Files\File;
 use OCP\Files\Folder;
@@ -299,8 +300,15 @@ protected function generalCreateChecks(IShare $share) {
 		$isFederatedShare = $share->getNode()->getStorage()->instanceOfStorage('\OCA\Files_Sharing\External\Storage');
 		$permissions = 0;
 
-		$userMounts = $userFolder->getById($share->getNode()->getId());
+		$userMounts = array_filter($userFolder->getById($share->getNode()->getId()), function($mount) {
+			// We need to filter since there might be other mountpoints that contain the file
+			// e.g. if the user has access to the same external storage that the file is originating from
+			return $mount->getStorage()->instanceOfStorage(ISharedStorage::class);
+		});
 		$userMount = array_shift($userMounts);
+		if ($userMount === null) {
+			throw new GenericShareException('Could not get proper share mount for ' . $share->getNode()->getId() . '. Failing since else the next calls are called with null');
+		}
 		$mount = $userMount->getMountPoint();
 		if (!$isFederatedShare && $share->getNode()->getOwner() && $share->getNode()->getOwner()->getUID() !== $share->getSharedBy()) {
 			// When it's a reshare use the parent share permissions as maximum