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Date:   Sat, 7 Sep 2019 03:13:35 +1000
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     Jeff Layton <jlayton@kernel.org>
Cc:     =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mickael.salaun@ssi.gouv.fr>,
        Florian Weimer <fweimer@redhat.com>,
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        Al Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Heimes <christian@python.org>,
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        Eric Chiang <ericchiang@google.com>,
        James Morris <jmorris@namei.org>, Jan Kara <jack@suse.cz>,
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        Mimi Zohar <zohar@linux.ibm.com>,
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        <philippe.trebuchet@ssi.gouv.fr>,
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        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()
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On 2019-09-06, Jeff Layton <jlayton@kernel.org> wrote:
> On Fri, 2019-09-06 at 18:06 +0200, Micka=EBl Sala=FCn wrote:
> > On 06/09/2019 17:56, Florian Weimer wrote:
> > > Let's assume I want to add support for this to the glibc dynamic load=
er,
> > > while still being able to run on older kernels.
> > >=20
> > > Is it safe to try the open call first, with O_MAYEXEC, and if that fa=
ils
> > > with EINVAL, try again without O_MAYEXEC?
> >=20
> > The kernel ignore unknown open(2) flags, so yes, it is safe even for
> > older kernel to use O_MAYEXEC.
> >=20
>=20
> 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).

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

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