RE: [RFC PATCH v4 2/3] git-compat-util: avoid failing dir ownership checks if running privileged

[<rsbecker@nexbridge.com>]

On May 9, 2022 12:55 PM, Junio C Hamano wrote:
>Carlo Marcelo Arenas Belón <carenas@gmail.com> writes:
>
>>> In exchange, the only attack scenario the code prevents is this,
>>> IIUC.
>>>
>>>  * You, the aspiring cracker, are a user not allowed to run "sudo" on
>>>    the box, and you know your uid is 1000
>>>
>>>  * You look for another user, a potential victim, whose uid is 1000
>>>    modulo 65536 (if your uid_t is 16-bit) and who can run "sudo" on
>>>    the box.
>>>
>>>  * You prepare a malicious repository, invite that user there and
>>>    ask them to run "sudo something" there.
>>>
>>> I'd say such an attack vector is not likely,...
>
>Sorry, I was totally wrong here.
>
>It is not just "not likely", but such an attack, with a potential victim not futzing with
>SUDO_UID environment themselves, would not work at all.  The value of
>SUDO_UID and the original uid of the potential victim by definition should fit in the
>uid_t type.  So if you, the aspiring cracker, have UID 1000, nobody else on the
>system has UID that is congruent modulo uid_t and wrap-around attack does not
>exist.  As long as the type we use to read SUDO_UID string into a variable is not
>narrower than uid_t, there.
>
>Of course you can tell any user who runs "sudo" to set SUDO_UID to
>1000 + 64k and cause wrap-around, but then you can tell them to set SUDO_UID to
>1000 without relying on wrap-around and have the same effect.  So, let's stop
>worrying about this bogus scenario.
>
>As to the "we can break compilation with -Wsign-compare on a system with signed
>uid_t", I agree that is true if we have
>
>	env_id <= (uid_t) -1
>
>there.  But I am not sure if that is the most effective way to smoke out platforms
>where this code has trouble working correctly.  Also, I would think that a system
>with signed uid_t is a possibility, but a user with a negative UID?
>
>I do not think even nobody4 was negative ;-)

I can test the code when it's ready.