linux-kernel - Re: [PATCH v7 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O

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Message-ID: <202007241203.0C596BB36@keescook>
Date:   Fri, 24 Jul 2020 12:03:21 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Eric Chiang <ericchiang@...gle.com>,
        Florian Weimer <fweimer@...hat.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Lakshmi Ramasubramanian <nramas@...ux.microsoft.com>,
        Matthew Garrett <mjg59@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Mimi Zohar <zohar@...ux.ibm.com>,
        Philippe Trébuchet 
        <philippe.trebuchet@....gouv.fr>,
        Scott Shell <scottsh@...rosoft.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Shuah Khan <shuah@...nel.org>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Thibaut Sautereau <thibaut.sautereau@...p-os.org>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH v7 5/7] fs,doc: Enable to enforce noexec mounts or file
 exec through O_MAYEXEC

On Thu, Jul 23, 2020 at 07:12:25PM +0200, Mickaël Salaün wrote:
> Allow for the enforcement of the O_MAYEXEC openat2(2) flag.  Thanks to
> the noexec option from the underlying VFS mount, or to the file execute
> permission, userspace can enforce these execution policies.  This may
> allow script interpreters to check execution permission before reading
> commands from a file, or dynamic linkers to allow shared object loading.
> 
> Add a new sysctl fs.open_mayexec_enforce to enable system administrators
> to enforce two complementary security policies according to the
> installed system: enforce the noexec mount option, and enforce
> executable file permission.  Indeed, because of compatibility with
> installed systems, only system administrators are able to check that
> this new enforcement is in line with the system mount points and file
> permissions.  A following patch adds documentation.
> 
> Being able to restrict execution also enables to protect the kernel by
> restricting arbitrary syscalls that an attacker could perform with a
> crafted binary or certain script languages.  It also improves multilevel
> isolation by reducing the ability of an attacker to use side channels
> with specific code.  These restrictions can natively be enforced for ELF
> binaries (with the noexec mount option) but require this kernel
> extension to properly handle scripts (e.g., Python, Perl).  To get a
> consistent execution policy, additional memory restrictions should also
> be enforced (e.g. thanks to SELinux).
> 
> Because the O_MAYEXEC flag is a meant to enforce a system-wide security
> policy (but not application-centric policies), it does not make sense
> for userland to check the sysctl value.  Indeed, this new flag only
> enables to extend the system ability to enforce a policy thanks to (some
> trusted) userland collaboration.  Moreover, additional security policies
> could be managed by LSMs.  This is a best-effort approach from the
> application developer point of view:
> https://lore.kernel.org/lkml/1477d3d7-4b36-afad-7077-a38f42322238@digikod.net/
> 
> Signed-off-by: Mickaël Salaün <mic@...ikod.net>

Acked-by: Kees Cook <keescook@...omium.org>

-- 
Kees Cook