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Date: Sat, 7 Sep 2019 08:18:00 +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 2019-09-07, Aleksa Sarai <cyphar@...har.com> wrote:
> On 2019-09-06, Jeff Layton <jlayton@...nel.org> wrote:
> > On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
> > > 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).
> > > 
> > 
> > I rather like the idea of having openat2 fds be non-executable by
> > default, and having userland request it specifically via O_MAYEXEC (or
> > some similar openat2 flag) if it's needed. Then you could add an
> > UPGRADE_EXEC flag instead?
> > 
> > That seems like something reasonable to do with a brand new API, and
> > might be very helpful for preventing certain classes of attacks.
> 
> In that case, maybe openat2(2) should default to not allowing any
> upgrades by default? The reason I pitched UPGRADE_NOEXEC is because
> UPGRADE_NO{READ,WRITE} are the existing @how->upgrade_mask flags.

Sorry, another issue is that there isn't a current way to really
restrict fexecve() permissions (from my [limited] understanding,
__FMODE_EXEC isn't the right thing to use) -- so we can't blanket block
exec through openat2() O_PATH descriptors and add UPGRADE_EXEC later.

We would have to implement FMODE_EXEC (and FMODE_MAP_EXEC as you
suggested) in order to implement FMODE_UPGRADE_EXEC before we could even
get a first version of openat2(2) in. Though, I do (a little
begrudgingly) agree that we should have a safe default if possible
(magical O_PATH reopening trickery is something that most people don't
know about and probably wouldn't want to happen if they did).

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

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