linux-kernel - Re: [PATCH v3 0/4] seccomp: Add SECCOMP_FILTER_FLAG_KILL

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Date:   Wed, 9 Aug 2017 13:40:08 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Tyler Hicks <tyhicks@...onical.com>
Cc:     Tycho Andersen <tycho@...ker.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Fabricio Voznika <fvoznika@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, Shuah Khan <shuah@...nel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 0/4] seccomp: Add SECCOMP_FILTER_FLAG_KILL_PROCESS

On Wed, Aug 9, 2017 at 1:33 PM, Tyler Hicks <tyhicks@...onical.com> wrote:
> Hey Tycho!
>
> On 08/09/2017 03:22 PM, Tycho Andersen wrote:
>> On Wed, Aug 09, 2017 at 12:01:53PM -0700, Kees Cook wrote:
>>> This series is the result of Fabricio and I going around a few times
>>> on possible solutions for finding a way to enhance RET_KILL to kill
>>> the process group. There's a lot of ways this could be done, but I
>>> wanted something that felt cleanest. As it happens, Tyler's recent
>>> patch series for logging improvement also needs to know a litte bit
>>> more during filter runs, and the solution for both is to pass back
>>> the matched filter. This lets us examine it here for RET_KILL and
>>> in the future for logging changes.
>>>
>>> The filter passing is patch 1, the new flag for RET_KILL is patch 2.
>>> Some test refactoring is in patch 3 for the RET_DATA ordering, and
>>> patch 4 is the test for the new RET_KILL flag.
>>>
>>> One thing missing is that CRIU will likely need to be updated, since
>>> saving/restoring seccomp filter _rules_ will not include the filter
>>> _flags_ for a process. This can be addressed separately.
>>
>> Thanks for the heads up, I suppose PTRACE_SECCOMP_GET_FLAGS similar to
>> how PTRACE_SECCOMP_GET_FILTER works will be fine for this. One
>> question is: would we then also need to keep track of the TSYNC flag?
>> I don't think CRIU needs this to be correct, and we can grab the
>> KILL_PROCESS flag from filter->kill_process, so perhaps it's moot.

It does not need to track TSYNC (since the results of that flag are
represented in the filter tree itself).

> Note that the logging changes that I'm working on also introduce a new
> filter flag (as Kees mentioned above). My filter flag is a lot like the
> KILL_PROCESS filter flag in that it is stored as a member of the
> seccomp_filter struct.
>
> I would think that you'd want to be able to do something like
> PTRACE_SECCOMP_GET_FILTER to (hopefully) future proof CRIU against all
> newly added filter flags.

I didn't see an obvious way to extend the existing
PTRACE_SECCOMP_GET_FILTER to also return flags, so I think either a
new function for just flags or a new versioned function for rules and
flags will be needed.

>> Anyway, happy to do this and the userspace part when this lands.

Okay, great! Thanks!

-Kees

-- 
Kees Cook
Pixel Security