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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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