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Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <e1ac9428e6b768ac3145aafbe19b24dd6cf410b9.camel@kernel.org>
Date:   Fri, 6 Sep 2019 13:06:00 -0700
Cc:     Aleksa Sarai <cyphar@cyphar.com>,
        =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mickael.salaun@ssi.gouv.fr>,
        Florian Weimer <fweimer@redhat.com>,
        =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <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>,
        =?utf-8?Q?Philippe_Tr=C3=A9buchet?= 
        <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>,
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References: <20190906152455.22757-1-mic@digikod.net> <20190906152455.22757-2-mic@digikod.net> <87ef0te7v3.fsf@oldenburg2.str.redhat.com> <75442f3b-a3d8-12db-579a-2c5983426b4d@ssi.gouv.fr> <f53ec45fd253e96d1c8d0ea6f9cca7f68afa51e3.camel@kernel.org> <20190906171335.d7mc3no5tdrcn6r5@yavin.dot.cyphar.com> <e1ac9428e6b768ac3145aafbe19b24dd6cf410b9.camel@kernel.org>
To:     Jeff Layton <jlayton@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Sep 6, 2019, at 12:43 PM, Jeff Layton <jlayton@kernel.org> wrote:
>=20
>> 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, Micka=C3=ABl Sala=C3=BCn wrote:
>>>>> On 06/09/2019 17:56, Florian Weimer wrote:
>>>>> Let's assume I want to add support for this to the glibc dynamic loade=
r,
>>>>> 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 fai=
ls
>>>>> 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.
>>=20
>> 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.
>>=20
>> 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.
>>=20
>>> 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.
>>=20
>> 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
>=20
> 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?
>=20
> That seems like something reasonable to do with a brand new API, and
> might be very helpful for preventing certain classes of attacks.
>=20
>=20

There are at least four concepts of executability here:

- Just check the file mode and any other relevant permissions. Return a norm=
al fd.  Makes sense for script interpreters, perhaps.

- Make the fd fexecve-able.

- Make the resulting fd mappable PROT_EXEC.

- Make the resulting fd upgradable.

I=E2=80=99m not at all convinced that the kernel needs to distinguish all th=
ese, but at least upgradability should be its own thing IMO.=