linux-kernel - Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and SHOULD_EXEC

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Message-ID: <ef3281ad-48a5-4316-b433-af285806540d@python.org>
Date: Mon, 8 Jul 2024 22:25:41 +0100
From: Steve Dower <steve.dower@...hon.org>
To: Jeff Xu <jeffxu@...gle.com>, Mickaël Salaün
 <mic@...ikod.net>
Cc: 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
Subject: Re: [RFC PATCH v19 2/5] security: Add new SHOULD_EXEC_CHECK and
 SHOULD_EXEC_RESTRICT securebits

On 08/07/2024 22:15, Jeff Xu wrote:
> IIUC:
> CHECK=0, RESTRICT=0: do nothing, current behavior
> CHECK=1, RESTRICT=0: permissive mode - ignore AT_CHECK results.
> CHECK=0, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, no exception.
> CHECK=1, RESTRICT=1: call AT_CHECK, deny if AT_CHECK failed, except
> those in the "checked-and-allowed" list.

I had much the same question for Mickaël while working on this.

Essentially, "CHECK=0, RESTRICT=1" means to restrict without checking. 
In the context of a script or macro interpreter, this just means it will 
never interpret any scripts. Non-binary code execution is fully disabled 
in any part of the process that respects these bits.

"CHECK=1, RESTRICT=1" means to restrict unless AT_CHECK passes. This 
case is the allow list (or whatever mechanism is being used to determine 
the result of an AT_CHECK check). The actual mechanism isn't the 
business of the script interpreter at all, it just has to refuse to 
execute anything that doesn't pass the check. So a generic interpreter 
can implement a generic mechanism and leave the specifics to whoever 
configures the machine.

The other two case are more obvious. "CHECK=0, RESTRICT=0" is the 
zero-overhead case, while "CHECK=1, RESTRICT=0" might log, warn, or 
otherwise audit the result of the check, but it won't restrict execution.

Cheers,
Steve