Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

Mickaël Salaün
2024-07-08T17:05:54Z

On Mon, Jul 08, 2024 at 09:40:45AM -0700, Jeff Xu wrote:
> On Mon, Jul 8, 2024 at 9:26 AM Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * Jeff Xu:
> >
> > > Will dynamic linkers use the execveat(AT_CHECK) to check shared
> > > libraries too ? or just the main executable itself.
> >
> > I expect that dynamic linkers will have to do this for everything they
> > map.

Correct, that would enable to safely handle LD_PRELOAD for instance.

> Then all the objects (.so, .sh, etc.) will go through the check from
> execveat's main to security_bprm_creds_for_exec(), some of them might
> be specific for the main executable ?
> e.g. ChromeOS uses security_bprm_creds_for_exec to block executable
> memfd [1], applying this means automatically extending the block to
> the .so object.

That's a good example of how this AT_CHECK check makes sense.

Landlock will probably get a similar (optional) restriction too:
https://github.com/landlock-lsm/linux/issues/37

>
> I'm not sure if other LSMs need to be updated ? e.g. will SELINUX
> check for .so with its process transaction policy ?

LSM should not need to be updated with this patch series. However,
systems/components/containers enabling this new check should make sure
it works with their current policy.

>
> [1] https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3834992
>
> -Jeff
>
>
> > Usually, that does not include the maim program, but this can
> > happen with explicit loader invocations (“ld.so /bin/true”).
> >
> > Thanks,
> > Florian
> >