Re: [PATCH v3 0/4] Improved seccomp logging

Andy Lutomirski
2017-05-02T16:14:41Z

On Mon, May 1, 2017 at 7:41 PM, Tyler Hicks <tyhicks@canonical.com> wrote:
> On 04/27/2017 07:42 PM, Kees Cook wrote:
>> On Thu, Apr 27, 2017 at 3:17 PM, Tyler Hicks <tyhicks@canonical.com> wrote:
>>> Quick update... I finished the move from the high-water mark
>>> log_max_action sysctl to the bitmask based actions_logged sysctl.
>>
>> Awesome!
>>
>>> Unfortunately, I've just realized that SECCOMP_SET_LOGGING, or any
>>> process-wide logging configuration mechanism, will not work. It is fine
>>> for the situation where two unrelated processes set up seccomp filters
>>> that should be logged differently. However, it fails when two closely
>>> related processes, such as parent and child, need to set up seccomp
>>> filters that should be logged differently. Imagine a launcher that sets
>>> up an application sandbox (including a seccomp filter) and then launches
>>> an electron app which will have its own seccomp filter for sandboxing
>>> untrusted code that it runs. Unless the launcher and app completely
>>> agree on actions that should be logged, the logging won't work as
>>> intended for both processes.
>>
>> Oh, you mean the forked process sets up the logging it wants for the
>> filters it just installed, then after exec a process sets up new
>> logging requirements?
>
> Yes - see below.
>
>>
>>> I think this needs to be configured at the filter level.
>>
>> I'm not sure that's even the right way to compose the logging desires.
>>
>> So, my initial thought was "whatever ran SECCOMP_SET_LOGGING knows
>> what it's doing" and it should be the actual value.
>>
>> If the launcher wants logs of everything the application does with its
>> filters, then a purely-tied-to-filter approach won't work either.
>>
>> Perhaps log bits can only be enabled? I.e. SECCOMP_SET_LOGGING
>> performs an OR instead of an assignment?
>
> The problem that I'm envisioning with this design is this:
>
> 1. Launcher is told to launch Chrome and forks off a process.
>
> 2. Launcher sets up a filter using RET_ERRNO for all unacceptable
> syscalls and enables auditing of RET_ERRNO.
>
> 3. Launcher execs Chrome.
>
> 4. Chrome then sets up its own, more restrictive filter that uses
> RET_ERRNO, among other actions, but does not want auditing of RET_ERRNO.
>
> If we use process-wide auditing controls, the logs will be filled with
> RET_ERRNO messages that were unintended and unrelated to the RET_ERRNO
> actions set up in the launcher's filter.
>
> Unfortunately, the OR'ing idea doesn't solve the problem.

Things like my more complicated solution solve this completely, I
think. The launcher would, by whatever means, say "RET_ERRNO and log
this". The more restrictive sandbox would say "RET_ERROR and don't
log this" and we'd just make sure that the composition rules mean the
inner rule wins.

--Andy