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From: Kees Cook <keescook@chromium.org>
To: Christian Heimes <christian@python.org>
Cc: "Mickaël Salaün" <mic@digikod.net>,
	linux-kernel@vger.kernel.org, "Aleksa Sarai" <cyphar@cyphar.com>,
	"Alexei Starovoitov" <ast@kernel.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Daniel Borkmann" <daniel@iogearbox.net>,
	"Deven Bowers" <deven.desai@linux.microsoft.com>,
	"Eric Chiang" <ericchiang@google.com>,
	"Florian Weimer" <fweimer@redhat.com>,
	"James Morris" <jmorris@namei.org>, "Jan Kara" <jack@suse.cz>,
	"Jann Horn" <jannh@google.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Lakshmi Ramasubramanian" <nramas@linux.microsoft.com>,
	"Matthew Garrett" <mjg59@google.com>,
	"Matthew Wilcox" <willy@infradead.org>,
	"Michael Kerrisk" <mtk.manpages@gmail.com>,
	"Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>,
	"Mimi Zohar" <zohar@linux.ibm.com>,
	"Philippe Trébuchet" <philippe.trebuchet@ssi.gouv.fr>,
	"Scott Shell" <scottsh@microsoft.com>,
	"Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Shuah Khan" <shuah@kernel.org>,
	"Steve Dower" <steve.dower@python.org>,
	"Steve Grubb" <sgrubb@redhat.com>,
	"Thibaut Sautereau" <thibaut.sautereau@ssi.gouv.fr>,
	"Vincent Strubel" <vincent.strubel@ssi.gouv.fr>,
	kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
	linux-integrity@vger.kernel.org,
	linux-security-module@vger.kernel.org,
	linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v5 1/6] fs: Add support for an O_MAYEXEC flag on openat2(2)
Date: Tue, 12 May 2020 15:56:46 -0700
Message-ID: <202005121555.0A446763@keescook> (raw)
In-Reply-To: <0c70debd-e79e-d514-06c6-4cd1e021fa8b@python.org>

On Tue, May 12, 2020 at 11:40:35PM +0200, Christian Heimes wrote:
> On 12/05/2020 23.05, Kees Cook wrote:
> > On Tue, May 05, 2020 at 05:31:51PM +0200, Mickaël Salaün wrote:
> >> When the O_MAYEXEC flag is passed, openat2(2) may be subject to
> >> additional restrictions depending on a security policy managed by the
> >> kernel through a sysctl or implemented by an LSM thanks to the
> >> inode_permission hook.  This new flag is ignored by open(2) and
> >> openat(2).
> >>
> >> The underlying idea is to be able to restrict scripts interpretation
> >> according to a policy defined by the system administrator.  For this to
> >> be possible, script interpreters must use the O_MAYEXEC flag
> >> appropriately.  To be fully effective, these interpreters also need to
> >> handle the other ways to execute code: command line parameters (e.g.,
> >> option -e for Perl), module loading (e.g., option -m for Python), stdin,
> >> file sourcing, environment variables, configuration files, etc.
> >> According to the threat model, it may be acceptable to allow some script
> >> interpreters (e.g. Bash) to interpret commands from stdin, may it be a
> >> TTY or a pipe, because it may not be enough to (directly) perform
> >> syscalls.  Further documentation can be found in a following patch.
> > 
> > You touch on this lightly in the cover letter, but it seems there are
> > plans for Python to restrict stdin parsing? Are there patches pending
> > anywhere for other interpreters? (e.g. does CLIP OS have such patches?)
> > 
> > There's always a push-back against adding features that have external
> > dependencies, and then those external dependencies can't happen without
> > the kernel first adding a feature. :) I like getting these catch-22s
> > broken, and I think the kernel is the right place to start, especially
> > since the threat model (and implementation) is already proven out in
> > CLIP OS, and now with IMA. So, while the interpreter side of this is
> > still under development, this gives them the tool they need to get it
> > done on the kernel side. So showing those pieces (as you've done) is
> > great, and I think finding a little bit more detail here would be even
> > better.
> 
> Hi,
> 
> Python core dev here.
> 
> Yes, there are plans to use feature for Python in combination with
> additional restrictions. For backwards compatibility reasons we cannot
> change the behavior of the default Python interpreter. I have plans to
> provide a restricted Python binary that prohibits piping from stdin,
> disables -c "some_code()", restricts import locations, and a couple of
> other things. O_MAYEXEC flag makes it easier to block imports from
> noexec filesystems.
> 
> My PoC [1] for a talk [2] last year is inspired by IMA appraisal and a
> previous talk by Mickaël on O_MAYEXEC.
> 
> Christian
> 
> [1] https://github.com/zooba/spython/blob/master/linux_xattr/spython.c
> [2]
> https://speakerdeck.com/tiran/europython-2019-auditing-hooks-and-security-transparency-for-cpython

Ah, fantastic; thank you! Yes, this will go a long way for helping
demonstration to other folks that there are people who will be using
this feature. :)

-- 
Kees Cook

  reply index

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-05 15:31 [PATCH v5 0/6] Add support for O_MAYEXEC Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 1/6] fs: Add support for an O_MAYEXEC flag on openat2(2) Mickaël Salaün
2020-05-12 21:05   ` Kees Cook
2020-05-12 21:40     ` Christian Heimes
2020-05-12 22:56       ` Kees Cook [this message]
2020-05-13 10:13     ` Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property Mickaël Salaün
2020-05-12 21:09   ` Kees Cook
2020-05-14  8:14     ` Lev R. Oshvang .
2020-05-14 15:48       ` Kees Cook
2020-05-17 16:57         ` Lev R. Oshvang .
2020-05-05 15:31 ` [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-05-05 15:44   ` Randy Dunlap
2020-05-05 16:55     ` Mickaël Salaün
2020-05-05 17:40       ` Randy Dunlap
2020-05-12 21:48   ` Kees Cook
2020-05-13 11:09     ` Mickaël Salaün
2020-05-13 15:37   ` Stephen Smalley
2020-05-13 23:27     ` Kees Cook
2020-05-14  3:05       ` Kees Cook
2020-05-14 10:12         ` David Laight
2020-05-14 12:22         ` Stephen Smalley
2020-05-14 14:41           ` Kees Cook
2020-05-14 15:52             ` Stephen Smalley
2020-05-14 15:45           ` Kees Cook
2020-05-14 16:10             ` Stephen Smalley
2020-05-14 19:16               ` Mickaël Salaün
2020-05-15  0:58                 ` Tetsuo Handa
2020-05-15  8:01                 ` How about just O_EXEC? (was Re: [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC) Kees Cook
2020-05-15  8:43                   ` Florian Weimer
2020-05-15 14:37                     ` Kees Cook
2020-05-15 14:43                       ` Florian Weimer
2020-05-15 15:50                         ` Kees Cook
2020-05-18  7:26                           ` Florian Weimer
2020-05-19  2:23                           ` Aleksa Sarai
2020-05-19 10:13                             ` Mickaël Salaün
2020-05-15 11:04                   ` Mickaël Salaün
2020-05-15 15:46                     ` Kees Cook
2020-05-15 18:24                       ` Mickaël Salaün
2020-05-14 19:21       ` [PATCH v5 3/6] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 4/6] selftest/openat2: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2020-05-12 21:57   ` Kees Cook
2020-05-13 11:18     ` Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 5/6] doc: Add documentation for the fs.open_mayexec_enforce sysctl Mickaël Salaün
2020-05-12 22:00   ` Kees Cook
2020-05-13 11:20     ` Mickaël Salaün
2020-05-05 15:31 ` [PATCH v5 6/6] ima: add policy support for the new file open MAY_OPENEXEC flag Mickaël Salaün
2020-05-05 15:36 ` [PATCH v5 0/6] Add support for O_MAYEXEC Mickaël Salaün
2020-05-06 13:58   ` Lev R. Oshvang .
2020-05-06 15:41     ` Aleksa Sarai
2020-05-07  8:30     ` Mickaël Salaün
2020-05-07  8:05 ` David Laight
2020-05-07  8:36   ` Mickaël Salaün
2020-05-07  9:00     ` David Laight
2020-05-07  9:30       ` Mickaël Salaün
2020-05-07  9:44         ` David Laight
2020-05-07 13:38           ` Mickaël Salaün
2020-05-08  7:15             ` Lev R. Oshvang .
2020-05-08 14:01               ` Mimi Zohar

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