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

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Date:   Mon, 9 Sep 2019 22:28:02 +1000
From:   Aleksa Sarai <cyphar@...har.com>
To:     Mickaël Salaün <mickael.salaun@....gouv.fr>
Cc:     James Morris <jmorris@...ei.org>, Jeff Layton <jlayton@...nel.org>,
        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>, 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 2019-09-09, Mickaël Salaün <mickael.salaun@....gouv.fr> wrote:
> On 09/09/2019 12:12, James Morris wrote:
> > On Mon, 9 Sep 2019, Mickaël Salaün wrote:
> >> As I said, O_MAYEXEC should be ignored if it is not supported by the
> >> kernel, which perfectly fit with the current open(2) flags behavior, and
> >> should also behave the same with openat2(2).
> >
> > The problem here is programs which are already using the value of
> > O_MAYEXEC, which will break.  Hence, openat2(2).
> 
> Well, it still depends on the sysctl, which doesn't enforce anything by
> default, hence doesn't break existing behavior, and this unused flags
> could be fixed/removed or reported by sysadmins or distro developers.

Okay, but then this means that new programs which really want to enforce
O_MAYEXEC (and know that they really do want this feature) won't be able
to unless an admin has set the relevant sysctl. Not to mention that the
old-kernel fallback will not cover the "it's disabled by the sysctl"
case -- so the fallback handling would need to be:

    int fd = open("foo", O_MAYEXEC|O_RDONLY);
    if (!(fcntl(fd, F_GETFL) & O_MAYEXEC))
        fallback();
    if (!sysctl_feature_is_enabled)
        fallback();

However, there is still a race here -- if an administrator enables
O_MAYEXEC after the program gets the fd, then you still won't hit the
fallback (and you can't tell that O_MAYEXEC checks weren't done).

You could fix the issue with the sysctl by clearing O_MAYEXEC from
f_flags if the sysctl is disabled. You could also avoid some of the
problems with it being a global setting by making it a prctl(2) which
processes can opt-in to (though this has its own major problems).

Sorry, but I'm just really not a fan of this.

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

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