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

Tyler Hicks
2017-04-10T19:22:28Z

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On 04/10/2017 10:57 AM, Andy Lutomirski wrote:
> On Fri, Apr 7, 2017 at 3:16 PM, Tyler Hicks <tyhicks-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org> wrote:
>> On 02/22/2017 12:46 PM, Kees Cook wrote:
>>> On Thu, Feb 16, 2017 at 3:29 PM, Kees Cook <keescook-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> wrote:
>>>> On Wed, Feb 15, 2017 at 7:24 PM, Andy Lutomirski <luto-kltTT9wpgjKXcx/E+B78Qg@public.gmane.orgt> wrote:
>>>>> On Mon, Feb 13, 2017 at 7:45 PM, Tyler Hicks <tyhicks-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org> wrote:
>>>>>> This patch set is the third revision of the following two previously
>>>>>> submitted patch sets:
>>>>>>
>>>>>> v1: http://lkml.kernel.org/r/1483375990-14948-1-git-send-email-tyhicks-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org
>>>>>> v1: http://lkml.kernel.org/r/1483377999-15019-2-git-send-email-tyhicks-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org
>>>>>>
>>>>>> v2: http://lkml.kernel.org/r/1486100262-32391-1-git-send-email-tyhicks-Z7WLFzj8eWMS+FvcfC7Uqw@public.gmane.org
>>>>>>
>>>>>> The patch set aims to address some known deficiencies in seccomp's current
>>>>>> logging capabilities:
>>>>>>
>>>>>> 1. Inability to log all filter actions.
>>>>>> 2. Inability to selectively enable filtering; e.g. devs want noisy logging,
>>>>>> users want relative quiet.
>>>>>> 3. Consistent behavior with audit enabled and disabled.
>>>>>> 4. Inability to easily develop a filter due to the lack of a
>>>>>> permissive/complain mode.
>>>>>
>>>>> I think I dislike this, but I think my dislikes may be fixable with
>>>>> minor changes.
>>>>>
>>>>> What I dislike is that this mixes app-specific built-in configuration
>>>>> (seccomp) with global privileged stuff (audit). The result is a
>>>>> potentially difficult to use situation in which you need to modify an
>>>>> app to make it loggable (using RET_LOG) and then fiddle with
>>>>> privileged config (auditctl, etc) to actually see the logs.
>>>>
>>>> You make a good point about RET_LOG vs log_max_action. I think making
>>>> RET_LOG the default value would work for 99% of the cases.
>>>
>>> Actually, I take this back: making "log" the default means that
>>> everything else gets logged too, include "expected" return values like
>>> errno, trap, etc. I think that would be extremely noisy as a default
>>> (for upstream or Ubuntu).
>>>
>>> Perhaps RET_LOG should unconditionally log? Or maybe the logged
>>> actions should be a bit field instead of a single value? Then the
>>> default could be "RET_KILL and RET_LOG", but an admin could switch it
>>> to just RET_KILL, or even nothing at all? Hmmm...
>>
>> Hi Kees - my apologies for going quiet on this topic after we spoke
>> about it more in IRC. I needed to tend to other work but now I'm able to
>> return to this seccomp logging patch set.
>>
>> To summarize what we discussed in IRC, the Chrome browser makes
>> extensive use of RET_ERRNO, RET_TRACE, etc., to sandbox code that may
>> not ever be adjusted to keep from bump into the sandbox walls.
>> Therefore, it is unreasonable to enable logging of something like
>> RET_ERRNO on a system-wide level where Chrome browser is in use.
>>
>> In contrast, snapd wants to set up "noisier" sandboxes for applications
>> to make it clear to the developers and the users that the sandboxed
>> application is bumping into the sandbox walls. Developers will know why
>> their code may not be working as intended and users will know that the
>> application is doing things that the platform doesn't agree with. These
>> sandboxes will end up using RET_ERRNO in the majority of cases.
>>
>> This means that with the current design of this patch set, Chrome
>> browser will either be unintentionally spamming the logs or snapd's
>> sandboxes will be helplessly silent when both Chrome and snapd is
>> installed at the same time, depending on the admin's preferences.
>>
>> To bring it back up a level, two applications may have a very different
>> outlook on how acceptable a given seccomp action is and they may
>> disagree on whether or not the user/administrator should be informed.
>>
>> I've been giving thought to the idea of providing a way for the
>> application setting up the filter to opt into logging of certain
>> actions. Here's a high level breakdown:
>
> At the risk of overcomplicating things, I think this issue may be a
> decent argument for doing something more like what I suggested
> earlier: let seccomp users emit loggable things and let their parents
> (optionally) catch and handle those things. Then we get real scoping
> rather than fiddling with bitmasks and hoping we get what we want to
> see without generating massive log spam.

Hi Andy - I remembered your proposal and considered it when I was
writing up mine. I still feel like it is easier to get the logging
bitmask right, if you even need to adjust the default bitmask, than to
force parent processes to act as a relay for log messages.

The logging bitmasks can easily be adjusted in debug builds or when a
user specifies a runtime option to enable seccomp logging in a program
that sets up filters.

Another potential issue with the parent process handling the logging is
that it isn't always possible when audit is in use.
audit_log_user_message() requires CAP_AUDIT_WRITE (seems like
CAP_AUDIT_CONTROL might also be required but I can't remember why) so
unprivileged processes would have to divert their log messages elsewhere.

Tyler

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