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

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Message-ID: <20190906170739.kk3opr2phidb7ilb@yavin.dot.cyphar.com>
Date:   Sat, 7 Sep 2019 03:07:39 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Mickaël Salaün <mickael.salaun@....gouv.fr>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Mickaël Salaün <mic@...ikod.net>,
        linux-kernel@...r.kernel.org, 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 2019-09-06, Mickaël Salaün <mickael.salaun@....gouv.fr> wrote:
> 
> 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.

Depends on your definition of "safe" -- a security feature that you will
silently not enable on older kernels doesn't sound super safe to me.
Unfortunately this is a limitation of open(2) that we cannot change --
which is why the openat2(2) proposal I've been posting gives -EINVAL for
unknown O_* flags.

There is a way to probe for support (though unpleasant), by creating a
test O_MAYEXEC fd and then checking if the flag is present in
/proc/self/fdinfo/$n.

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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