apparmor: fix potential label refcnt leak in aa_change_profile

aa_change_profile() invokes aa_get_current_label(), which returns
a reference of the current task's label.

According to the comment of aa_get_current_label(), the returned
reference must be put with aa_put_label().
However, when the original object pointed by "label" becomes
unreachable because aa_change_profile() returns or a new object
is assigned to "label", reference count increased by
aa_get_current_label() is not decreased, causing a refcnt leak.

Fix this by calling aa_put_label() before aa_change_profile() return
and dropping unnecessary aa_get_current_label().

Fixes: 9fcf78cc ("apparmor: update domain transitions that are subsets of confinement at nnp")
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/domain.c

@@ -1328,6 +1328,7 @@ int aa_change_profile(const char *fqname, int flags)
 		ctx->nnp = aa_get_label(label);
 
 	if (!fqname || !*fqname) {
+		aa_put_label(label);
 		AA_DEBUG("no profile name");
 		return -EINVAL;
 	}
@@ -1346,8 +1347,6 @@ int aa_change_profile(const char *fqname, int flags)
 			op = OP_CHANGE_PROFILE;
 	}
 
-	label = aa_get_current_label();
-
 	if (*fqname == '&') {
 		stack = true;
 		/* don't have label_parse() do stacking */