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

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Message-ID: <CALCETrWHKga33bvzUHnd-mRQUeNXTtXSS8Y8+40d5bxv-CqBhw@mail.gmail.com>
Date: Wed, 27 Aug 2025 17:32:02 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Aleksa Sarai <cyphar@...har.com>
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>, 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 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.

IPython says:

In [1]: import mmap, tempfile

In [2]: f = tempfile.TemporaryFile()

In [3]: f.write(b'initial contents')
Out[3]: 16

In [4]: f.flush()

In [5]: map = mmap.mmap(f.fileno(), f.tell(), flags=mmap.MAP_PRIVATE,
prot=mmap.PROT_READ)

In [6]: map[:]
Out[6]: b'initial contents'

In [7]: f.seek(0)
Out[7]: 0

In [8]: f.write(b'changed')
Out[8]: 7

In [9]: f.flush()

In [10]: map[:]
Out[10]: b'changed contents'