linux-kernel - Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys

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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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