apparmor: Fix aa_label refcnt leak in policy_update

policy_update() invokes begin_current_label_crit_section(), which
returns a reference of the updated aa_label object to "label" with
increased refcount.

When policy_update() returns, "label" becomes invalid, so the refcount
should be decreased to keep refcount balanced.

The reference counting issue happens in one exception handling path of
policy_update(). When aa_may_manage_policy() returns not NULL, the
refcnt increased by begin_current_label_crit_section() is not decreased,
causing a refcnt leak.

Fix this issue by jumping to "end_section" label when
aa_may_manage_policy() returns not NULL.

Fixes: 5ac8c355 ("apparmor: allow introspecting the loaded policy pre internal transform")
Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/apparmorfs.c

@@ -454,7 +454,7 @@ static ssize_t policy_update(u32 mask, const char __user *buf, size_t size,
 	 */
 	error = aa_may_manage_policy(label, ns, mask);
 	if (error)
-		return error;
+		goto end_section;
 
 	data = aa_simple_write_to_buffer(buf, size, size, pos);
 	error = PTR_ERR(data);
@@ -462,6 +462,7 @@ static ssize_t policy_update(u32 mask, const char __user *buf, size_t size,
 		error = aa_replace_profiles(ns, label, mask, data);
 		aa_put_loaddata(data);
 	}
+end_section:
 	end_current_label_crit_section(label);
 
 	return error;