linux-kernel - Re: [RFC PATCH v19 0/5] Script execution control (was O

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Message-ID: <20240716.leeV4ooveinu@digikod.net>
Date: Tue, 16 Jul 2024 19:31:38 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Roberto Sassu <roberto.sassu@...weicloud.com>, 
	Mimi Zohar <zohar@...ux.ibm.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>, 
	James Morris <jamorris@...ux.microsoft.com>, Jan Kara <jack@...e.cz>, Jann Horn <jannh@...gle.com>, 
	Jeff Xu <jeffxu@...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>, Nicolas Bouchinet <nicolas.bouchinet@....gouv.fr>, 
	Scott Shell <scottsh@...rosoft.com>, Shuah Khan <shuah@...nel.org>, 
	Stephen Rothwell <sfr@...b.auug.org.au>, Steve Dower <steve.dower@...hon.org>, 
	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
Subject: Re: [RFC PATCH v19 0/5] Script execution control (was O_MAYEXEC)

On Tue, Jul 16, 2024 at 12:12:49PM -0400, James Bottomley wrote:
> On Tue, 2024-07-16 at 17:57 +0200, Roberto Sassu wrote:
> > But the Clip OS 4 patch does not cover the redirection case:
> > 
> > # ./bash < /root/test.sh
> > Hello World
> > 
> > Do you have a more recent patch for that?

Bash was only partially restricted for CLIP OS because it was used for
administrative tasks (interactive shell).

Python was also restricted for user commands though:
https://github.com/clipos-archive/clipos4_portage-overlay/blob/master/dev-lang/python/files/python-2.7.9-clip-mayexec.patch

Steve and Christian could help with a better Python implementation.

> 
> How far down the rabbit hole do you want to go?  You can't forbid a
> shell from executing commands from stdin because logging in then won't
> work.  It may be possible to allow from a tty backed file and not from
> a file backed one, but you still have the problem of the attacker
> manually typing in the script.

Yes, that's why we'll have the (optional) SECBIT_EXEC_DENY_INTERACTIVE:
https://lore.kernel.org/all/20240710.eiKohpa4Phai@digikod.net/

> 
> The saving grace for this for shells is that they pretty much do
> nothing on their own (unlike python) so you can still measure all the
> executables they call out to, which provides reasonable safety.

Exactly. Python is a much more interesting target for attacker because
it opens the door for arbitrary syscalls (see the cover letter).

If we want to have a more advanced access control (e.g. allow Bash but
not Python), we should extend existing LSMs to manage the appropriate
securebits according to programs/subjects.