Re: [kernel-hardening] [PATCH 00/11] S.A.R.A. a new stacked LSM

[Matt Brown <matt@nmatt.com>]

On 7/11/17 12:58 PM, Salvatore Mesoraca wrote:
> 2017-07-11 1:40 GMT+02:00 Mickaël Salaün <mic@digikod.net>:
>>
>> On 10/07/2017 09:59, Salvatore Mesoraca wrote:
>>> 2017-07-09 21:35 GMT+02:00 Mickaël Salaün <mic@digikod.net>:
>>>> Hi,
>>>>
>>>> I think it make sense to merge the W^X features with the TPE/shebang LSM
>>>> [1].
>>>>
>>>> Regards,
>>>>  Mickaël
>>>>
>>>> [1]
>>>> https://lkml.kernel.org/r/d9aca46b-97c6-4faf-b559-484feb4aa640@digikod.net
>>>
>>> Hi,
>>> Can you elaborate why it would be an advantage to have those features merged?
>>> They seem quite unrelated.
>>> Also, they work in rather different ways in respect to how they are configured.
>>> I'm not sure what would be a reasonable way to merge them.
>>> Thank you for your comment,
>>>
>>> Salvatore
>>>
>>
>> The aim of the Trusted Path Execution is to constraint calls to execve
>> (e.g. forbid an user to execute his own binaries, i.e. apply a W^X
>> security policy). This should handle binaries and could handle scripts
>> too [1]. However, there is always a way for a process to mmap/mprotect
>> arbitrary data and make it executable, be it intentional or not. PaX and
>> the W^X part of your LSM can handle this, or make exceptions by marking
>> a file with dedicated xattr values. This kind of exception fit well with
>> TPE to get a more hardened executable security policy (e.g. forbid an
>> user to execute his own binaries or to mmap arbitrary executable code).
>> Moreover, TPE could handle some part of its configuration from some
>> xattr values (e.g. allow scripts/interpreters, a whitelist of
>> environment variables, additional memory restrictions…) as you do with
>> SARA thanks to your tools.
> 
> I understand your point. They complement each other in some sense.
> On the other hand, I'm still worried about the suitability of merging,
> under the same LSM, two features that are managed in two
> completely different ways.
> IMHO, if they have to be merged, they should be "integrated".
> As I see it, there are only 3 possible solutions to this problem:
> 1 - SARA gives up its configuration mechanics and starts using xattrs
> 2 - TPE/shebang gives up xattrs and starts using SARA-style configurations
> 3 - SARA adds xattrs support to its quiver *and* TPE/shebang adds SARA-style
>      configuration support.
> 
> The solution number 1 is the one I'm less inclined to, as you can imagine.
> I'm in favor of solutions 2 and 3, but of course we need to know Mimi Zohar and
> Matt Brown opinion on this matter.
> If we can find a consensus on the best way to merge them, I'm not against
> the merge.
> Anyway, these LSMs are stackable and they can be used together even if they
> don't get merged. So I think that merging them is not a "must".
> 
> Salvatore
> 

I have merged my TPE LSM with Mimi Zohar's shebang LSM and will be
releasing a version 3 soon. I have also added securityfs support to
shebang that will allow users to update the interpreter list at run
time. This allows for user's to configure TPE/Shebang without any
xattrs. For a preview of my version 3 you can check out my dev tree
here:
https://github.com/nmatt0/linux-security/tree/tpe/security/tpe

Note: that git tree is WIP and may not have all of the attribution and
documentation needed.

Matt Brown