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Date:   Mon, 9 Sep 2019 22:28:02 +1000
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mickael.salaun@ssi.gouv.fr>
Cc:     James Morris <jmorris@namei.org>, Jeff Layton <jlayton@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mic@digikod.net>,
        linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Heimes <christian@python.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Eric Chiang <ericchiang@google.com>, Jan Kara <jack@suse.cz>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Matthew Garrett <mjg59@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Philippe =?utf-8?Q?Tr=C3=A9buchet?= 
        <philippe.trebuchet@ssi.gouv.fr>,
        Scott Shell <scottsh@microsoft.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Shuah Khan <shuah@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Steve Dower <steve.dower@python.org>,
        Steve Grubb <sgrubb@redhat.com>,
        Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>,
        Vincent Strubel <vincent.strubel@ssi.gouv.fr>,
        Yves-Alexis Perez <yves-alexis.perez@ssi.gouv.fr>,
        kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
        linux-security-module@vger.kernel.org,
        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()
Message-ID: <20190909122802.imfx6wp4zeroktuz@yavin>
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On 2019-09-09, Micka=EBl Sala=FCn <mickael.salaun@ssi.gouv.fr> wrote:
> On 09/09/2019 12:12, James Morris wrote:
> > On Mon, 9 Sep 2019, Micka=EBl Sala=FCn 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, a=
nd
> >> 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).
>=20
> 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 =3D 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.

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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