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From: Aleksa Sarai <cyphar@cyphar.com>
To: Jeff Layton <jlayton@kernel.org>
Cc: "Mickaël Salaün" <mickael.salaun@ssi.gouv.fr>,
	"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>,
	"James Morris" <jmorris@namei.org>, "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: Sat, 7 Sep 2019 08:18:00 +1000
Message-ID: <20190906221800.xzowcjgdeljikcyu@yavin.dot.cyphar.com> (raw)
In-Reply-To: <20190906220546.gkqxzm4y3ynevvtz@yavin.dot.cyphar.com>

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On 2019-09-07, Aleksa Sarai <cyphar@cyphar.com> wrote:
> On 2019-09-06, Jeff Layton <jlayton@kernel.org> wrote:
> > On Sat, 2019-09-07 at 03:13 +1000, Aleksa Sarai wrote:
> > > On 2019-09-06, Jeff Layton <jlayton@kernel.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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  reply index

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-06 15:24 [PATCH v2 0/5] Add support for O_MAYEXEC Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open() Mickaël Salaün
2019-09-06 15:56   ` Florian Weimer
2019-09-06 16:06     ` Mickaël Salaün
2019-09-06 16:48       ` Jeff Layton
2019-09-06 17:13         ` Aleksa Sarai
2019-09-06 19:43           ` Jeff Layton
2019-09-06 20:06             ` Andy Lutomirski
2019-09-06 20:51               ` Jeff Layton
2019-09-06 21:27                 ` Andy Lutomirski
2019-09-06 22:12                 ` Aleksa Sarai
2019-09-09  9:33               ` Mickaël Salaün
2019-09-06 22:05             ` Aleksa Sarai
2019-09-06 22:18               ` Aleksa Sarai [this message]
2019-09-06 17:14         ` Mickaël Salaün
2019-09-06 18:38           ` Jeff Layton
2019-09-06 18:41             ` Andy Lutomirski
2019-09-09  9:18               ` Mickaël Salaün
2019-09-09 15:49                 ` Andy Lutomirski
2019-09-06 18:44             ` Florian Weimer
2019-09-06 19:03             ` James Morris
2019-09-09  9:25               ` Mickaël Salaün
2019-09-09 10:12                 ` James Morris
2019-09-09 10:54                   ` Mickaël Salaün
2019-09-09 12:28                     ` Aleksa Sarai
2019-09-09 12:33                       ` Mickaël Salaün
2019-09-09 11:54                 ` Aleksa Sarai
2019-09-09 12:28                   ` Mickaël Salaün
2019-09-06 17:07       ` Aleksa Sarai
2019-09-06 17:20         ` Christian Brauner
2019-09-06 17:24           ` Mickaël Salaün
2019-09-06 17:40           ` Tycho Andersen
2019-09-06 18:27             ` Florian Weimer
2019-09-06 18:46               ` Tycho Andersen
2019-09-06 15:24 ` [PATCH v2 2/5] fs: Add a MAY_EXECMOUNT flag to infer the noexec mount propertie Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 3/5] fs: Enable to enforce noexec mounts or file exec through O_MAYEXEC Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 4/5] selftest/exec: Add tests for O_MAYEXEC enforcing Mickaël Salaün
2019-09-06 15:24 ` [PATCH v2 5/5] doc: Add documentation for the fs.open_mayexec_enforce sysctl Mickaël Salaün
2019-09-06 18:50 ` [PATCH v2 0/5] Add support for O_MAYEXEC Steve Grubb
2019-09-06 18:57   ` Florian Weimer
2019-09-06 19:07     ` Steve Grubb
2019-09-06 19:26       ` Andy Lutomirski
2019-09-06 22:44         ` Aleksa Sarai
2019-09-09  9:09           ` Mickaël Salaün
2019-09-09  0:16 ` James Morris

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