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

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Message-ID: <a0bddc6f-686e-2a5a-4833-acf04a10722f@canonical.com>
Date:   Wed, 9 Aug 2017 15:33:28 -0500
From:   Tyler Hicks <tyhicks@...onical.com>
To:     Tycho Andersen <tycho@...ker.com>,
        Kees Cook <keescook@...omium.org>
Cc:     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>,
        linux-kselftest@...r.kernel.org,
        linux-security-module@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH v3 0/4] seccomp: Add SECCOMP_FILTER_FLAG_KILL_PROCESS

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.

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'll also mention that I have a libseccomp branch in the making that
allows libseccomp to query the kernel to see if it supports a given
filter flag. I haven't done a PR on that yet because I'm waiting to see
how my related kernel patches play out (they seem to be getting close to
being acceptable).

Tyler

> 
> Anyway, happy to do this and the userspace part when this lands.
> 
> Cheers,
> 
> Tycho
> 
>> Please take a look!
>>
>> Thanks,
>>
>> -Kees
>>
>> v3:
>> - adjust seccomp_run_filters() to avoid later filters from masking
>>   kill-process RET_KILL actions (drewry)
>> - add test for masked RET_KILL.
>>
>> v2:
>> - moved kill_process bool into struct padding gap (tyhicks)
>> - improved comments/docs in various places for clarify (tyhicks)
>> - use ASSERT_TRUE() for WIFEXITED and WIFSIGNALLED (tyhicks)
>> - adding Reviewed-bys from tyhicks
>>




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