linux-kernel - Re: [RFC PATCH v1 1/2] fs: Add O_DENY

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Message-ID: <lhuikibbv0g.fsf@oldenburg.str.redhat.com>
Date: Mon, 25 Aug 2025 11:39:11 +0200
From: Florian Weimer <fweimer@...hat.com>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Andy Lutomirski <luto@...capital.net>,  Jann Horn <jannh@...gle.com>,
  Al Viro <viro@...iv.linux.org.uk>,  Christian Brauner
 <brauner@...nel.org>,  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>,  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,  Jeff Xu <jeffxu@...omium.org>
Subject: Re: [RFC PATCH v1 1/2] fs: Add O_DENY_WRITE

* Mickaël Salaün:

> The order of checks would be:
> 1. open script with O_DENY_WRITE
> 2. check executability with AT_EXECVE_CHECK
> 3. read the content and interpret it
>
> The deny-write feature was to guarantee that there is no race condition
> between step 2 and 3.  All these checks are supposed to be done by a
> trusted interpreter (which is allowed to be executed).  The
> AT_EXECVE_CHECK call enables the caller to know if the kernel (and
> associated security policies) allowed the *current* content of the file
> to be executed.  Whatever happen before or after that (wrt.
> O_DENY_WRITE) should be covered by the security policy.

Why isn't it an improper system configuration if the script file is
writable?

In the past, the argument was that making a file (writable and)
executable was an auditable even, and that provided enough coverage for
those people who are interested in this.

Thanks,
Florian