Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on sys_open()

Mickaël Salaün
2019-09-09T12:34:12Z

On 09/09/2019 14:28, Aleksa Sarai wrote:
> On 2019-09-09, Mickaël Salaün <mickael.salaun@ssi.gouv.fr> wrote:
>> On 09/09/2019 12:12, James Morris wrote:
>>> On Mon, 9 Sep 2019, Mickaël Salaün 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, 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.
>
> 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 = 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/

--
Mickaël Salaün

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