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Date: Mon, 9 Sep 2019 11:33:40 +0200
From: Mickaël Salaün <mickael.salaun@....gouv.fr>
To: Andy Lutomirski <luto@...capital.net>, Jeff Layton <jlayton@...nel.org>
CC: Aleksa Sarai <cyphar@...har.com>, 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 06/09/2019 22:06, Andy Lutomirski wrote:
>
>
>> On Sep 6, 2019, at 12:43 PM, 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.
>>
>>
>
> There are at least four concepts of executability here:
>
> - Just check the file mode and any other relevant permissions. Return a normal fd.  Makes sense for script interpreters, perhaps.

This is the purpose of this patch series. It doesn't make sense to add
memory restrictions nor constrain fexecve and such.


>
> - Make the fd fexecve-able.
>
> - Make the resulting fd mappable PROT_EXEC.
>
> - Make the resulting fd upgradable.
>
> I’m not at all convinced that the kernel needs to distinguish all these, but at least upgradability should be its own thing IMO.
>

--
Mickaël Salaün

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