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

Andy Lutomirski
2024-07-18T01:03:10Z

> 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
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.

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