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

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Message-ID: <CALCETrUM7LAzm-pXwgTVACvO7uvaUZ0B7j=6Vy0BrDkngpu=yg@mail.gmail.com>
Date: Mon, 1 Sep 2025 09:01:15 -0700
From: Andy Lutomirski <luto@...nel.org>
To: Jann Horn <jannh@...gle.com>
Cc: "Serge E. Hallyn" <serge@...lyn.com>, Andy Lutomirski <luto@...nel.org>, 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>, 
	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>, 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 Mon, Sep 1, 2025 at 4:06 AM Jann Horn <jannh@...gle.com> wrote:
>
> On Thu, Aug 28, 2025 at 11:01 PM Serge E. Hallyn <serge@...lyn.com> wrote:
> > On Wed, Aug 27, 2025 at 05:32:02PM -0700, Andy Lutomirski 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.
> > >
> > > 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'
> >
> > That was surprising to me, however, if I split the reader
> > and writer into different processes, so
>
> Testing this in python is a terrible idea because it obfuscates the
> actual syscalls from you.
>
> > P1:
> > f = open("/tmp/3", "w")
> > f.write('initial contents')
> > f.flush()
> >
> > P2:
> > import mmap
> > f = open("/tmp/3", "r")
> > map = mmap.mmap(f.fileno(), f.tell(), flags=mmap.MAP_PRIVATE, prot=mmap.PROT_READ)
> >
> > Back to P1:
> > f.seek(0)
> > f.write('changed')
> >
> > Back to P2:
> > map[:]
> >
> > Then P2 gives me:
> >
> > b'initial contents'
>
> Because when you executed `f.write('changed')`, Python internally
> buffered the write. "changed" is never actually written into the file
> in your example. If you add a `f.flush()` in P1 after this, running
> `map[:]` in P2 again will show you the new data.
>

These days, one can type in Python, ask an LLM to translate to C, and
get almost-correct output :)  Or one can use os.write(), which is
exactly what I should have done.

--Andy