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Date: Sat, 7 Sep 2019 03:13:35 +1000
From: Aleksa Sarai <cyphar@...har.com>
To: Jeff Layton <jlayton@...nel.org>
Cc: Mickaël Salaün <mickael.salaun@....gouv.fr>,
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, Jeff Layton <jlayton@...nel.org> wrote:
> On Fri, 2019-09-06 at 18:06 +0200, Mickaël Salaün 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.
> >
>
> Well...maybe. What about existing programs that are sending down bogus
> open flags? Once you turn this on, they may break...or provide a way to
> circumvent the protections this gives.
It should be noted that this has been a valid concern for every new O_*
flag introduced (and yet we still introduced new flags, despite the
concern) -- though to be fair, O_TMPFILE actually does have a
work-around with the O_DIRECTORY mask setup.
The openat2() set adds O_EMPTYPATH -- though in fairness it's also
backwards compatible because empty path strings have always given ENOENT
(or EINVAL?) while O_EMPTYPATH is a no-op non-empty strings.
> Maybe this should be a new flag that is only usable in the new openat2()
> syscall that's still under discussion? That syscall will enforce that
> all flags are recognized. You presumably wouldn't need the sysctl if you
> went that route too.
I'm also interested in whether we could add an UPGRADE_NOEXEC flag to
how->upgrade_mask for the openat2(2) patchset (I reserved a flag bit for
it, since I'd heard about this work through the grape-vine).
--
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>
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