I'm not sure whether this is the right place to write this, but those permission checks look really racy - both the file type test and the file owner UID check.
(Besides, I think that an attacker should be able to hardlink a file created by another user into his directory, which would also lead to a bypass of the UID equality check, but I'm not familiar enough with ubuntu to be able to say whether that attack is actually feasible.)
I'm not sure whether this is the right place to write this, but those permission checks look really racy - both the file type test and the file owner UID check.
(Besides, I think that an attacker should be able to hardlink a file created by another user into his directory, which would also lead to a bypass of the UID equality check, but I'm not familiar enough with ubuntu to be able to say whether that attack is actually feasible.)