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

Mickaël Salaün
2024-07-18T12:23:04Z

On Thu, Jul 18, 2024 at 09:02:56AM +0800, Andy Lutomirski wrote:
> > On Jul 17, 2024, at 6:01 PM, Mickaël Salaün <mic@digikod.net> wrote:
> >
> > On Wed, Jul 17, 2024 at 09:26:22AM +0100, Steve Dower wrote:
> >>> On 17/07/2024 07:33, Jeff Xu wrote:
> >>> Consider those cases: I think:
> >>> a> relying purely on userspace for enforcement does't seem to be
> >>> effective, e.g. it is trivial to call open(), then mmap() it into
> >>> executable memory.
> >>
> >> If there's a way to do this without running executable code that had to pass
> >> a previous execveat() check, then yeah, it's not effective (e.g. a Python
> >> interpreter that *doesn't* enforce execveat() is a trivial way to do it).
> >>
> >> Once arbitrary code is running, all bets are off. So long as all arbitrary
> >> code is being checked itself, it's allowed to do things that would bypass
> >> later checks (and it's up to whoever audited it in the first place to
> >> prevent this by not giving it the special mark that allows it to pass the
> >> check).
> >
> > Exactly. As explained in the patches, one crucial prerequisite is that
> > the executable code is trusted, and the system must provide integrity
> > guarantees. We cannot do anything without that. This patches series is
> > a building block to fix a blind spot on Linux systems to be able to
> > fully control executability.
>
> Circling back to my previous comment (did that ever get noticed?), I

Yes, I replied to your comments. Did I miss something?

> don’t think this is quite right:
>
> https://lore.kernel.org/all/CALCETrWYu=PYJSgyJ-vaa+3BGAry8Jo8xErZLiGR3U5h6+U0tA@mail.gmail.com/
>
> On a basic system configuration, a given path either may or may not be
> executed. And maybe that path has some integrity check (dm-verity,
> etc). So the kernel should tell the interpreter/loader whether the
> target may be executed. All fine.
>
> But I think the more complex cases are more interesting, and the
> “execute a program” process IS NOT BINARY. An attempt to execute can
> be rejected outright, or it can be allowed *with a change to creds or
> security context*. It would be entirely reasonable to have a policy
> that allows execution of non-integrity-checked files but in a very
> locked down context only.

I guess you mean to transition to a sandbox when executing an untrusted
file. This is a good idea. I talked about role transition in the
patch's description:

With the information that a script interpreter is about to interpret a
script, an LSM security policy can adjust caller's access rights or log
execution request as for native script execution (e.g. role transition).
This is possible thanks to the call to security_bprm_creds_for_exec().

>
> So… shouldn’t a patch series to this effect actually support this?
>

This patch series brings the minimal building blocks to have a
consistent execution environment. Role transitions for script execution
are left to LSMs. For instance, we could extend Landlock to
automatically sandbox untrusted scripts.