[RFC PATCH v1 3/5] Yama: Enforces noexec mounts or file executability through O_MAYEXEC

[Mickaël Salaün]

On 09/01/2019 00:30, Kees Cook wrote:
> On Tue, Jan 8, 2019 at 5:29 AM Mickaël Salaün
> <mickael.salaun at ssi.gouv.fr> wrote:
>>
>>
>> On 03/01/2019 12:17, Jann Horn wrote:
>>> On Thu, Dec 13, 2018 at 3:49 PM Mickaël Salaün
>>> <mickael.salaun at ssi.gouv.fr> wrote:
>>>> On 12/12/2018 18:09, Jann Horn wrote:
>>>>> On Wed, Dec 12, 2018 at 9:18 AM Mickaël Salaün <mic at digikod.net> wrote:
>>>>>> Enable to either propagate the mount options from the underlying VFS
>>>>>> mount to prevent execution, or to propagate the file execute permission.
>>>>>> This may allow a script interpreter to check execution permissions
>>>>>> before reading commands from a file.
>>>>>>
>>>>>> The main goal is to be able to protect the kernel by restricting
>>>>>> arbitrary syscalls that an attacker could perform with a crafted binary
>>>>>> or certain script languages.  It also improves multilevel isolation
>>>>>> by reducing the ability of an attacker to use side channels with
>>>>>> specific code.  These restrictions can natively be enforced for ELF
>>>>>> binaries (with the noexec mount option) but require this kernel
>>>>>> extension to properly handle scripts (e.g., Python, Perl).
> 
> I like this idea, but I think it shouldn't live in Yama (since it is
> currently intended to be a ptrace-policy-only LSM). It was
> _originally_ designed to do various DAC improvements, but the
> agreement was that those should live directly in the VFS instead (i.e.
> the symlink, hardlink and now fifo and regular file defenses).
> 
> This should likely go in similarly. (But if not, it could also be its own LSM.)
> 

I think that Yama is quite handy and make sense here, but I'm fine
putting this knob elsewhere. However, I was thinking, for a future patch
series, to add another sysctl to lock this choice, i.e. generalizing the
way Yama can lock the ptrace_scope.

What matter here is the ability for an LSM to use this O_MAYEXEC flag.
Yama is a good place to showcase this feature and I think it is cleaner
to leverage the LSM framework to put new (optional) security features. I
can easily create a new LSM but it would be pretty similar to Yama...
What do you think about it James and Al?

Side question: wouldn't it be better to use a 0600 mode (instead of
0644) for this kind of sysctl?