Message-ID: <20240718.ahph4che5Shi@digikod.net>
Date: Thu, 18 Jul 2024 14:23:19 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Jeff Xu <jeffxu@...gle.com>
Cc: Steve Dower <steve.dower@...hon.org>, 
	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>, 
	James Morris <jamorris@...ux.microsoft.com>, Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, 
	Jonathan Corbet <corbet@....net>, Jordan R Abrahams <ajordanr@...gle.com>, 
	Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>, Luca Boccassi <bluca@...ian.org>, 
	Luis Chamberlain <mcgrof@...nel.org>, "Madhavan T . Venkataraman" <madvenka@...ux.microsoft.com>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Matthew Garrett <mjg59@...f.ucam.org>, 
	Matthew Wilcox <willy@...radead.org>, Miklos Szeredi <mszeredi@...hat.com>, 
	Mimi Zohar <zohar@...ux.ibm.com>, Nicolas Bouchinet <nicolas.bouchinet@....gouv.fr>, 
	Scott Shell <scottsh@...rosoft.com>, Shuah Khan <shuah@...nel.org>, 
	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 Wed, Jul 17, 2024 at 06:51:11PM -0700, Jeff Xu wrote:
> On Wed, Jul 17, 2024 at 3:00 AM 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).
> >
> We will want to define what is considered as "arbitrary code is running"
> 
> Using an example of ROP, attackers change the return address in stack,
> e.g. direct the execution flow to a gauge to call "ld.so /tmp/a.out",
> do you consider "arbitrary code is running" when stack is overwritten
> ? or after execve() is called.

Yes, ROP is arbitrary code execution (which can be mitigated with CFI).
ROP could be enough to interpret custom commands and create a small
interpreter/VM.

> If it is later, this patch can prevent "ld.so /tmp/a.out".
> 
> > 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.
> 
> Even trusted executable can have a bug.

Definitely, but this patch series is dedicated to script execution
control.

> 
> I'm thinking in the context of ChromeOS, where all its system services
> are from trusted partitions, and legit code won't load .so from a
> non-exec mount.  But we want to sandbox those services, so even under
> some kind of ROP attack, the service still won't be able to load .so
> from /tmp. Of course, if an attacker can already write arbitrary
> length of data into the stack, it is probably already a game over.
> 

OK, you want to tie executable file permission to mmap.  That makes
sense if you have a consistent execution model.  This can be enforced by
LSMs.  Contrary to script interpretation which is a full user space
implementation (and then controlled by user space), mmap restrictions
should indeed be enforced by the kernel.

Powered by blists - more mailing lists