apparmor: Fix aa_label_build() error handling for failed merges

[ Upstream commit d6d478ae ]

aa_label_merge() can return NULL for memory allocations failures
make sure to handle and set the correct error in this case.
Reported-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>

security/apparmor/domain.c

@@ -1260,7 +1260,10 @@ int aa_change_profile(const char *fqname, int flags)
 					aa_get_label(&profile->label));
 		if (IS_ERR_OR_NULL(new)) {
 			info = "failed to build target label";
-			error = PTR_ERR(new);
+			if (!new)
+				error = -ENOMEM;
+			else
+				error = PTR_ERR(new);
 			new = NULL;
 			perms.allow = 0;
 			goto audit;