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Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()
To:     Aleksa Sarai <cyphar@cyphar.com>
CC:     James Morris <jmorris@namei.org>, Jeff Layton <jlayton@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>,
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References: <20190906152455.22757-2-mic@digikod.net>
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 <75442f3b-a3d8-12db-579a-2c5983426b4d@ssi.gouv.fr>
 <f53ec45fd253e96d1c8d0ea6f9cca7f68afa51e3.camel@kernel.org>
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 <alpine.LRH.2.21.1909061202070.18660@namei.org>
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 <alpine.LRH.2.21.1909090309260.27895@namei.org>
 <073cb831-7c6b-1882-9b7d-eb810a2ef955@ssi.gouv.fr>
 <20190909122802.imfx6wp4zeroktuz@yavin>
From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mickael.salaun@ssi.gouv.fr>
Message-ID: <045db737-107f-d06a-04ea-fdd0758de062@ssi.gouv.fr>
Date:   Mon, 9 Sep 2019 14:33:51 +0200
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On 09/09/2019 14:28, Aleksa Sarai wrote:
> On 2019-09-09, Micka=EBl Sala=FCn <mickael.salaun@ssi.gouv.fr> wrote:
>> On 09/09/2019 12:12, James Morris wrote:
>>> On Mon, 9 Sep 2019, Micka=EBl Sala=FCn wrote:
>>>> 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, a=
nd
>>>> 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.
>
> Okay, but then this means that new programs which really want to enforce
> O_MAYEXEC (and know that they really do want this feature) won't be able
> to unless an admin has set the relevant sysctl. Not to mention that the
> old-kernel fallback will not cover the "it's disabled by the sysctl"
> case -- so the fallback handling would need to be:
>
>     int fd =3D open("foo", O_MAYEXEC|O_RDONLY);
>     if (!(fcntl(fd, F_GETFL) & O_MAYEXEC))
>         fallback();
>     if (!sysctl_feature_is_enabled)
>         fallback();
>
> However, there is still a race here -- if an administrator enables
> O_MAYEXEC after the program gets the fd, then you still won't hit the
> fallback (and you can't tell that O_MAYEXEC checks weren't done).

I just replied to this concern here:
https://lore.kernel.org/lkml/70e4244e-4dfb-6e67-416b-445e383aa1b5@ssi.gouv.=
fr/

>
> You could fix the issue with the sysctl by clearing O_MAYEXEC from
> f_flags if the sysctl is disabled. You could also avoid some of the
> problems with it being a global setting by making it a prctl(2) which
> processes can opt-in to (though this has its own major problems).

Security definition and enforcement should be manageable by sysadmins
and distro developers.

>
> Sorry, but I'm just really not a fan of this.

I guess there is some misunderstanding. I just replied to another thread
and I think it should answer your concerns (especially about the PDP and
PEP):
https://lore.kernel.org/lkml/70e4244e-4dfb-6e67-416b-445e383aa1b5@ssi.gouv.=
fr/


--
Micka=EBl Sala=FCn

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