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

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Message-ID: <CALCETrUtJmWxKYSi6QQAGpQR_ETNfoBidCu_VEq8Lx9iJAOyEw@mail.gmail.com>
Date: Mon, 1 Sep 2025 09:25:09 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Roberto Sassu <roberto.sassu@...weicloud.com>
Cc: Aleksa Sarai <cyphar@...har.com>, Mickaël Salaün <mic@...ikod.net>, 
	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)

Can you clarify this a bit for those of us who are not well-versed in
exactly what "measurement" does?

On Mon, Sep 1, 2025 at 2:42 AM Roberto Sassu
<roberto.sassu@...weicloud.com> wrote:
> > Now, in cases where you have IMA or something and you only permit signed
> > binaries to execute, you could argue there is a different race here (an
> > attacker creates a malicious script, runs it, and then replaces it with
> > a valid script's contents and metadata after the fact to get
> > AT_EXECVE_CHECK to permit the execution). However, I'm not sure that
>
> Uhm, let's consider measurement, I'm more familiar with.
>
> I think the race you wanted to express was that the attacker replaces
> the good script, verified with AT_EXECVE_CHECK, with the bad script
> after the IMA verification but before the interpreter reads it.
>
> Fortunately, IMA is able to cope with this situation, since this race
> can happen for any file open, where of course a file can be not read-
> locked.

I assume you mean that this has nothing specifically to do with
scripts, as IMA tries to protect ordinary (non-"execute" file access)
as well.  Am I right?

>
> If the attacker tries to concurrently open the script for write in this
> race window, IMA will report this event (called violation) in the
> measurement list, and during remote attestation it will be clear that
> the interpreter did not read what was measured.
>
> We just need to run the violation check for the BPRM_CHECK hook too
> (then, probably for us the O_DENY_WRITE flag or alternative solution
> would not be needed, for measurement).

This seems consistent with my interpretation above, but ...

>
> Please, let us know when you apply patches like 2a010c412853 ("fs:
> don't block i_writecount during exec"). We had a discussion [1], but
> probably I missed when it was decided to be applied (I saw now it was
> in the same thread, but didn't get that at the time). We would have
> needed to update our code accordingly. In the future, we will try to
> clarify better our expectations from the VFS.

... I didn't follow this.

Suppose there's some valid contents of /bin/sleep.  I execute
/bin/sleep 1m.  While it's running, I modify /bin/sleep (by opening it
for write, not by replacing it), and the kernel in question doesn't do
ETXTBSY.  Then the sleep process reads (and executes) the modified
contents.  Wouldn't a subsequent attestation fail?  Why is ETXTBSY
needed?