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Date: Mon, 9 Sep 2019 12:54:16 +0200
From: Mickaël Salaün <mickael.salaun@....gouv.fr>
To: James Morris <jmorris@...ei.org>
CC: Jeff Layton <jlayton@...nel.org>, Florian Weimer <fweimer@...hat.com>,
	Mickaël Salaün <mic@...ikod.net>,
	<linux-kernel@...r.kernel.org>, Aleksa Sarai <cyphar@...har.com>, 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>, 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 09/09/2019 12:12, James Morris wrote:
> On Mon, 9 Sep 2019, Mickaël Salaün wrote:
>
>>
>> On 06/09/2019 21:03, James Morris wrote:
>>> On Fri, 6 Sep 2019, Jeff Layton wrote:
>>>
>>>> The fact that open and openat didn't vet unknown flags is really a bug.
>>>>
>>>> Too late to fix it now, of course, and as Aleksa points out, we've
>>>> worked around that in the past. Now though, we have a new openat2
>>>> syscall on the horizon. There's little need to continue these sorts of
>>>> hacks.
>>>>
>>>> New open flags really have no place in the old syscalls, IMO.
>>>
>>> Agree here. It's unfortunate but a reality and Linus will reject any such
>>> changes which break existing userspace.
>>
>> Do you mean that adding new flags to open(2) is not possible?
>>
>> Does it means that unspecified behaviors are definitely part of the
>> Linux specification and can't be fixed?
>
> This is my understanding.
>
>>
>> As I said, O_MAYEXEC should be ignored if it is not supported by the
>> kernel, which perfectly fit with the current open(2) flags behavior, and
>> should also behave the same with openat2(2).
>
> The problem here is programs which are already using the value of
> O_MAYEXEC, which will break.  Hence, openat2(2).

Well, it still depends on the sysctl, which doesn't enforce anything by
default, hence doesn't break existing behavior, and this unused flags
could be fixed/removed or reported by sysadmins or distro developers.


--
Mickaël Salaün

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