[PATCH] security/commoncap: don't assume "setid" if all ids are identical

[Eric W. Biederman]

Max Kellermann <max.kellermann at ionos.com> writes:

> On Fri, May 9, 2025 at 12:12 AM <sergeh at kernel.org> wrote:
>> ABI stability is about the most important thing to Linus, so yes, if
>> documentation and code disagree, then we should fix the documentation,
>> except in the case where the current behavior just really is wrong
>> or insecure.
>
> It is insecure indeed (can be abused for LD_PRELOAD
> attacks):https://lore.kernel.org/lkml/CAKPOu+8+1uVrDJHwmHJd2d46-N6AwjR4_bbtoSJS+sx6J=rkjg@mail.gmail.com/

I don't understand what you are trying to solve,
but the patch at the top of the thread introduces a
has_identical_uids_gids and is pure nonsense.

In particular __is_setuid or __is_setgid being true guarantees
that has_identical_uids_gids will be false.

Which means has_identical_uids_gids adds nothing, and the patch is
pointless.



If your concern is LD_PRELOAD and the like please don't play with
the uids/gids and instead just make certain bprm->secureexec gets
set.



At this point I am pretty certain that changing the logic and leaving
extra uids/gids set will result in security vulnerabilities for someone
who actually depends upon how the code works today.  I see no evidence
in this conversation that anyone has surveyed the users of NO_NEW_PRIVS
and verified how anyone actually uses it.  Without such evidence we
have to assume that userspace depends upon the current behavior.

Eric