fuse: add a flag FUSE_SETXATTR_ACL_KILL_SGID to kill SGID

When posix access ACL is set, it can have an effect on file mode and it can
also need to clear SGID if.

- None of caller's group/supplementary groups match file owner group.
AND
- Caller is not priviliged (No CAP_FSETID).

As of now fuser server is responsible for changing the file mode as
well. But it does not know whether to clear SGID or not.

So add a flag FUSE_SETXATTR_ACL_KILL_SGID and send this info with SETXATTR
to let file server know that sgid needs to be cleared as well.
Reported-by: Luis Henriques <lhenriques@suse.de>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/acl.c

@@ -71,6 +71,7 @@ int fuse_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
 		return -EINVAL;
 
 	if (acl) {
+		unsigned int extra_flags = 0;
 		/*
 		 * Fuse userspace is responsible for updating access
 		 * permissions in the inode, if needed. fuse_setxattr
@@ -94,7 +95,11 @@ int fuse_set_acl(struct user_namespace *mnt_userns, struct inode *inode,
 			return ret;
 		}
 
-		ret = fuse_setxattr(inode, name, value, size, 0, 0);
+		if (!in_group_p(i_gid_into_mnt(&init_user_ns, inode)) &&
+		    !capable_wrt_inode_uidgid(&init_user_ns, inode, CAP_FSETID))
+			extra_flags |= FUSE_SETXATTR_ACL_KILL_SGID;
+
+		ret = fuse_setxattr(inode, name, value, size, 0, extra_flags);
 		kfree(value);
 	} else {
 		ret = fuse_removexattr(inode, name);

include/uapi/linux/fuse.h

@@ -180,6 +180,7 @@
  *  - add FUSE_HANDLE_KILLPRIV_V2, FUSE_WRITE_KILL_SUIDGID, FATTR_KILL_SUIDGID
  *  - add FUSE_OPEN_KILL_SUIDGID
  *  - extend fuse_setxattr_in, add FUSE_SETXATTR_EXT
+ *  - add FUSE_SETXATTR_ACL_KILL_SGID
  */
 
 #ifndef _LINUX_FUSE_H
@@ -454,6 +455,12 @@ struct fuse_file_lock {
  */
 #define FUSE_OPEN_KILL_SUIDGID	(1 << 0)
 
+/**
+ * setxattr flags
+ * FUSE_SETXATTR_ACL_KILL_SGID: Clear SGID when system.posix_acl_access is set
+ */
+#define FUSE_SETXATTR_ACL_KILL_SGID	(1 << 0)
+
 enum fuse_opcode {
 	FUSE_LOOKUP		= 1,
 	FUSE_FORGET		= 2,  /* no reply */