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

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Message-ID: <20190906184620.GI7627@cisco>
Date:   Fri, 6 Sep 2019 12:46:20 -0600
From:   Tycho Andersen <tycho@...ho.ws>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Christian Brauner <christian.brauner@...ntu.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Mickaël Salaün <mickael.salaun@....gouv.fr>,
        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 Fri, Sep 06, 2019 at 08:27:31PM +0200, Florian Weimer wrote:
> * Tycho Andersen:
> 
> > On Fri, Sep 06, 2019 at 07:20:51PM +0200, Christian Brauner wrote:
> >> On Sat, Sep 07, 2019 at 03:07:39AM +1000, Aleksa Sarai wrote:
> >> > 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.
> >> 
> >> Which Florian said they can't do for various reasons.
> >> 
> >> It is a major painpoint if there's no easy way for userspace to probe
> >> for support. Especially if it's security related which usually means
> >> that you want to know whether this feature works or not.
> >
> > What about just trying to violate the policy via fexecve() instead of
> > looking around in /proc? Still ugly, though.
> 
> How would we do this?  This is about opening the main executable as part
> of an explicit loader invocation.  Typically, an fexecve will succeed
> and try to run the program, but with the wrong dynamic loader.

Yeah, fexecve() was a think-o, sorry, you don't need to go that far. I
was thinking do what the tests in this series do: create a tmpfs with
MS_NOEXEC, put an executable file in it, and try and open it with
O_MAYEXEC. If that works, the kernel doesn't support the flag, and it
should give you -EACCES if the kernel does support the flag.

Still a lot of work, though. Seems better to just use openat2.

Tycho