linux-kernel - Re: [RFC PATCH v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE

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Message-ID: <20250826.iewie7Et5aiw@digikod.net>
Date: Tue, 26 Aug 2025 19:47:30 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Theodore Ts'o <tytso@....edu>
Cc: Christian Brauner <brauner@...nel.org>, 
	Al Viro <viro@...iv.linux.org.uk>, Kees Cook <keescook@...omium.org>, 
	Paul Moore <paul@...l-moore.com>, Serge Hallyn <serge@...lyn.com>, 
	Andy Lutomirski <luto@...nel.org>, Arnd Bergmann <arnd@...db.de>, 
	Christian Heimes <christian@...hon.org>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Elliott Hughes <enh@...gle.com>, Fan Wu <wufan@...ux.microsoft.com>, 
	Florian Weimer <fweimer@...hat.com>, 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>, 
	Matt Bobrowski <mattbobrowski@...gle.com>, Miklos Szeredi <mszeredi@...hat.com>, 
	Mimi Zohar <zohar@...ux.ibm.com>, Nicolas Bouchinet <nicolas.bouchinet@....cyber.gouv.fr>, 
	Robert Waite <rowait@...rosoft.com>, Roberto Sassu <roberto.sassu@...wei.com>, 
	Scott Shell <scottsh@...rosoft.com>, Steve Dower <steve.dower@...hon.org>, 
	Steve Grubb <sgrubb@...hat.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 v1 0/2] Add O_DENY_WRITE (complement AT_EXECVE_CHECK)

On Tue, Aug 26, 2025 at 08:30:41AM -0400, Theodore Ts'o wrote:
> Is there a single, unified design and requirements document that
> describes the threat model, and what you are trying to achieve with
> AT_EXECVE_CHECK and O_DENY_WRITE?  I've been looking at the cover
> letters for AT_EXECVE_CHECK and O_DENY_WRITE, and the documentation
> that has landed for AT_EXECVE_CHECK and it really doesn't describe
> what *are* the checks that AT_EXECVE_CHECK is trying to achieve:
> 
>    "The AT_EXECVE_CHECK execveat(2) flag, and the
>    SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE
>    securebits are intended for script interpreters and dynamic linkers
>    to enforce a consistent execution security policy handled by the
>    kernel."

>From the documentation:

  Passing the AT_EXECVE_CHECK flag to execveat(2) only performs a check
  on a regular file and returns 0 if execution of this file would be
  allowed, ignoring the file format and then the related interpreter
  dependencies (e.g. ELF libraries, script’s shebang).

> 
> Um, what security policy?

Whether the file is allowed to be executed.  This includes file
permission, mount point option, ACL, LSM policies...

> What checks?

Executability checks?

> What is a sample exploit
> which is blocked by AT_EXECVE_CHECK?

Executing/interpreting any data: sh script.txt

> 
> And then on top of it, why can't you do these checks by modifying the
> script interpreters?

The script interpreter requires modification to use AT_EXECVE_CHECK.

There is no other way for user space to reliably check executability of
files (taking into account all enforced security
policies/configurations).