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Message-ID: <aLDDk4x7QBKxLmoi@mail.hallyn.com>
Date: Thu, 28 Aug 2025 16:01:07 -0500
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Andy Lutomirski <luto@...nel.org>
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>, 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 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
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'
-serge
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