linux-kernel - Re: [RFC PATCH v19 1/5] exec: Add a new AT

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Message-ID: <CALCETrUcr3p_APNazMro7Y9FX1zLAiQESvKZ5BDgd8X3PoCdFw@mail.gmail.com>
Date: Thu, 18 Jul 2024 09:02:56 +0800
From: Andy Lutomirski <luto@...capital.net>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Steve Dower <steve.dower@...hon.org>, Jeff Xu <jeffxu@...gle.com>, 
	Al Viro <viro@...iv.linux.org.uk>, Christian Brauner <brauner@...nel.org>, 
	Kees Cook <keescook@...omium.org>, Linus Torvalds <torvalds@...ux-foundation.org>, 
	Paul Moore <paul@...l-moore.com>, "Theodore Ts'o" <tytso@....edu>, Alejandro Colomar <alx@...nel.org>, 
	Aleksa Sarai <cyphar@...har.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Casey Schaufler <casey@...aufler-ca.com>, Christian Heimes <christian@...hon.org>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Eric Biggers <ebiggers@...nel.org>, 
	Eric Chiang <ericchiang@...gle.com>, Fan Wu <wufan@...ux.microsoft.com>, 
	Florian Weimer <fweimer@...hat.com>, Geert Uytterhoeven <geert@...ux-m68k.org>, 
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	Vincent Strubel <vincent.strubel@....gouv.fr>, Xiaoming Ni <nixiaoming@...wei.com>, 
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	linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	linux-integrity@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

> On Jul 17, 2024, at 6:01 PM, Mickaël Salaün <mic@...ikod.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?