Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

From: Aleksa Sarai <cyphar@cyphar.com>
To: "Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>
Cc: "James Morris" <jmorris@namei.org>,
	"Jeff Layton" <jlayton@kernel.org>,
	"Florian Weimer" <fweimer@redhat.com>,
	"Mickaël Salaün" <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 Trébuchet" <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()
Date: Mon, 9 Sep 2019 22:28:02 +1000	[thread overview]
Message-ID: <20190909122802.imfx6wp4zeroktuz@yavin> (raw)
In-Reply-To: <073cb831-7c6b-1882-9b7d-eb810a2ef955@ssi.gouv.fr>

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On 2019-09-09, Mickaël Salaün <mickael.salaun@ssi.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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