linux-kernel - Re: [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount property

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Date:   Thu, 14 May 2020 08:48:57 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Lev R. Oshvang ." <levonshe@...il.com>
Cc:     Mickaël Salaün <mic@...ikod.net>,
        linux-kernel@...r.kernel.org, Aleksa Sarai <cyphar@...har.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Deven Bowers <deven.desai@...ux.microsoft.com>,
        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>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        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>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        kernel-hardening@...ts.openwall.com, linux-api@...r.kernel.org,
        linux-integrity@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v5 2/6] fs: Add a MAY_EXECMOUNT flag to infer the noexec
 mount property

On Thu, May 14, 2020 at 11:14:04AM +0300, Lev R. Oshvang . wrote:
> New sysctl is indeed required to allow userspace that places scripts
> or libs under noexec mounts.

But since this is a not-uncommon environment, we must have the sysctl
otherwise this change would break those systems.

> fs.mnt_noexec_strict =0 (allow, e) , 1 (deny any file with --x
> permission), 2 (deny when O_MAYEXEC absent), for any file with ---x
> permissions)

I don't think we want another mount option -- this is already fully
expressed with noexec and the system-wide sysctl.

-- 
Kees Cook