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

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Message-ID: <20240718.Niexoo0ahch0@digikod.net>
Date: Thu, 18 Jul 2024 14:22:49 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Andy Lutomirski <luto@...capital.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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	Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>, Luca Boccassi <bluca@...ian.org>, 
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	Stephen Rothwell <sfr@...b.auug.org.au>, Steve Grubb <sgrubb@...hat.com>, 
	Thibaut Sautereau <thibaut.sautereau@....gouv.fr>, Vincent Strubel <vincent.strubel@....gouv.fr>, 
	Xiaoming Ni <nixiaoming@...wei.com>, Yin Fengwei <fengwei.yin@...el.com>, 
	kernel-hardening@...ts.openwall.com, 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, Elliott Hughes <enh@...gle.com>
Subject: Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

On Thu, Jul 18, 2024 at 09:02:56AM +0800, Andy Lutomirski wrote:
> > 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

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.