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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <202407100921.687BE1A6@keescook>
Date: Wed, 10 Jul 2024 09:26:14 -0700
From: Kees Cook <kees@...nel.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Jeff Xu <jeffxu@...gle.com>, Steve Dower <steve.dower@...hon.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Christian Brauner <brauner@...nel.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 Wed, Jul 10, 2024 at 11:58:25AM +0200, Mickaël Salaün wrote:
> Here is another proposal:
> 
> We can change a bit the semantic by making it the norm to always check
> file executability with AT_CHECK, and using the securebits to restrict
> file interpretation and/or command injection (e.g. user supplied shell
> commands).  Non-executable checked files can be reported/logged at the
> kernel level, with audit, configured by sysadmins.
> 
> New securebits (feel free to propose better names):
> 
> - SECBIT_EXEC_RESTRICT_FILE: requires AT_CHECK to pass.

Would you want the enforcement of this bit done by userspace or the
kernel?

IIUC, userspace would always perform AT_CHECK regardless of
SECBIT_EXEC_RESTRICT_FILE, and then which would happen?

1) userspace would ignore errors from AT_CHECK when
   SECBIT_EXEC_RESTRICT_FILE is unset

or

2) kernel would allow all AT_CHECK when SECBIT_EXEC_RESTRICT_FILE is
   unset

I suspect 1 is best and what you intend, given that
SECBIT_EXEC_DENY_INTERACTIVE can only be enforced by userspace.

> - SECBIT_EXEC_DENY_INTERACTIVE: deny any command injection via
>   command line arguments, environment variables, or configuration files.
>   This should be ignored by dynamic linkers.  We could also have an
>   allow-list of shells for which this bit is not set, managed by an
>   LSM's policy, if the native securebits scoping approach is not enough.
> 
> Different modes for script interpreters:
> 
> 1. RESTRICT_FILE=0 DENY_INTERACTIVE=0 (default)
>    Always interpret scripts, and allow arbitrary user commands.
>    => No threat, everyone and everything is trusted, but we can get
>    ahead of potential issues with logs to prepare for a migration to a
>    restrictive mode.
> 
> 2. RESTRICT_FILE=1 DENY_INTERACTIVE=0
>    Deny script interpretation if they are not executable, and allow
>    arbitrary user commands.
>    => Threat: (potential) malicious scripts run by trusted (and not
>       fooled) users.  That could protect against unintended script
>       executions (e.g. sh /tmp/*.sh).
>    ==> Makes sense for (semi-restricted) user sessions.
> 
> 3. RESTRICT_FILE=1 DENY_INTERACTIVE=1
>    Deny script interpretation if they are not executable, and also deny
>    any arbitrary user commands.
>    => Threat: malicious scripts run by untrusted users.
>    ==> Makes sense for system services executing scripts.
> 
> 4. RESTRICT_FILE=0 DENY_INTERACTIVE=1
>    Always interpret scripts, but deny arbitrary user commands.
>    => Goal: monitor/measure/assess script content (e.g. with IMA/EVM) in
>       a system where the access rights are not (yet) ready.  Arbitrary
>       user commands would be much more difficult to monitor.
>    ==> First step of restricting system services that should not
>        directly pass arbitrary commands to shells.

I like these bits!

-- 
Kees Cook