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

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Message-ID: <2025-08-28-alert-groggy-mugs-lapse-IWqeR7@cyphar.com>
Date: Thu, 28 Aug 2025 10:52:42 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: 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>, 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 2025-08-27, Andy Lutomirski <luto@...nel.org> wrote:
> On Wed, Aug 27, 2025 at 5:14 PM Aleksa Sarai <cyphar@...har.com> wrote:
> >
> > On 2025-08-26, Mickaël Salaün <mic@...ikod.net> wrote:
> > > On Tue, Aug 26, 2025 at 11:07:03AM +0200, Christian Brauner wrote:
> > > > Nothing has changed in that regard and I'm not interested in stuffing
> > > > the VFS APIs full of special-purpose behavior to work around the fact
> > > > that this is work that needs to be done in userspace. Change the apps,
> > > > stop pushing more and more cruft into the VFS that has no business
> > > > there.
> > >
> > > It would be interesting to know how to patch user space to get the same
> > > guarantees...  Do you think I would propose a kernel patch otherwise?
> >
> > You could mmap the script file with MAP_PRIVATE. This is the *actual*
> > protection the kernel uses against overwriting binaries (yes, ETXTBSY is
> > nice but IIRC there are ways to get around it anyway).
> 
> Wait, really?  MAP_PRIVATE prevents writes to the mapping from
> affecting the file, but I don't think that writes to the file will
> break the MAP_PRIVATE CoW if it's not already broken.

Oh I guess you're right -- that's news to me. And from mmap(2):

> MAP_PRIVATE
> [...] It is unspecified whether changes made to the file after the
> mmap() call are visible in the mapped region.

But then what is the protection mechanism (in the absence of -ETXTBSY)
that stops you from overwriting the live text of a binary by just
writing to it?

I would need to go trawling through my old scripts to find the
reproducer that let you get around -ETXTBSY (I think it involved
executable memfds) but I distinctly remember that even if you overwrote
the binary you would not see the live process's mapped mm change value.
(Ditto for the few kernels when we removed -ETXTBSY.) I found this
surprising, but assumed that it was because of MAP_PRIVATE.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
https://www.cyphar.com/

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