linux-kernel - Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys

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Message-ID: <75442f3b-a3d8-12db-579a-2c5983426b4d@ssi.gouv.fr>
Date:   Fri, 6 Sep 2019 18:06:25 +0200
From:   Mickaël Salaün <mickael.salaun@....gouv.fr>
To:     Florian Weimer <fweimer@...hat.com>,
        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>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Heimes <christian@...hon.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Chiang <ericchiang@...gle.com>,
        James Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        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>,
        Song Liu <songliubraving@...com>,
        Steve Dower <steve.dower@...hon.org>,
        Steve Grubb <sgrubb@...hat.com>,
        Thibaut Sautereau <thibaut.sautereau@....gouv.fr>,
        Vincent Strubel <vincent.strubel@....gouv.fr>,
        Yves-Alexis Perez <yves-alexis.perez@....gouv.fr>,
        <kernel-hardening@...ts.openwall.com>, <linux-api@...r.kernel.org>,
        <linux-security-module@...r.kernel.org>,
        <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()

On 06/09/2019 17:56, Florian Weimer wrote:
> Let's assume I want to add support for this to the glibc dynamic loader,
> while still being able to run on older kernels.
>
> Is it safe to try the open call first, with O_MAYEXEC, and if that fails
> with EINVAL, try again without O_MAYEXEC?

The kernel ignore unknown open(2) flags, so yes, it is safe even for
older kernel to use O_MAYEXEC.

>
> Or do I risk disabling this security feature if I do that?

It is only a security feature if the kernel support it, otherwise it is
a no-op.

>
> Do we need a different way for recognizing kernel support.  (Note that
> we cannot probe paths in /proc for various reasons.)

There is no need to probe for kernel support.

>
> Thanks,
> Florian
>

--
Mickaël Salaün

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