apparmor: fix reference count leak in aa_pivotroot()

The aa_pivotroot() function has a reference counting bug in a specific
path. When aa_replace_current_label() returns on success, the function
forgets to decrement the reference count of “target”, which is
increased earlier by build_pivotroot(), causing a reference leak.

Fix it by decreasing the refcount of “target” in that path.

Fixes: 2ea3ffb7 ("apparmor: add mount mediation")
Co-developed-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Co-developed-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: Xin Xiong <xiongx18@fudan.edu.cn>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/mount.c

@@ -719,6 +719,7 @@ int aa_pivotroot(struct aa_label *label, const struct path *old_path,
 			aa_put_label(target);
 			goto out;
 		}
+		aa_put_label(target);
 	} else
 		/* already audited error */
 		error = PTR_ERR(target);