[PATCH v3 0/4] Improved seccomp logging

Fri Apr 7 23:46:17 UTC 2017

On 04/07/2017 05:46 PM, Kees Cook wrote:
> On Fri, Apr 7, 2017 at 3:16 PM, Tyler Hicks <tyhicks at canonical.com> wrote:
>> On 02/22/2017 12:46 PM, Kees Cook wrote:
>>> On Thu, Feb 16, 2017 at 3:29 PM, Kees Cook <keescook at chromium.org> wrote:
>>>> On Wed, Feb 15, 2017 at 7:24 PM, Andy Lutomirski <luto at amacapital.net> wrote:
>>>>> On Mon, Feb 13, 2017 at 7:45 PM, Tyler Hicks <tyhicks at canonical.com> 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@canonical.com
>>>>>> v1: http://lkml.kernel.org/r/1483377999-15019-2-git-send-email-tyhicks@canonical.com
>>>>>>
>>>>>> v2: http://lkml.kernel.org/r/1486100262-32391-1-git-send-email-tyhicks@canonical.com
>>>>>>
>>>>>> 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:
>>
>>  - The administrator will have control of system-wide seccomp logging
>>    and the application will have control of how its seccomp actions are
>>    logged
>>  - Both the administrator's and application's logging preferences are
>>    bitmasks of actions that should be logged
>>  - The default administrator bitmask is set to log all actions except
>>    RET_ALLOW
>>    + Configurable via a sysctl
>>    + Very similar to the log_max_action syscall that was proposed in
>>      this patch set but a bitmask instead of a max action
>>  - The default application bitmask is set to log only RET_KILL and
>>    RET_LOG
>>    + Configurable via the seccomp(2) syscall (details TBD)
>>  - Actions are only logged when the application has requested the
>>    action to be logged and the administrator has allowed the action to
>>    be logged
>>    + By default, this is only RET_KILL and RET_LOG actions
>>
>> Let me know what you think about this and I can turn out another patch
>> set next week if it sounds reasonable.
> 
> This seems good to me. With a bitmask we don't have to play crazy
> games with levels, and with the app able to declare what it wants
> logged, we get the flexibility needed by app developers.
> 
> One change I think I'd like to make is that an app can't block
> RET_KILL from being logged (the sysadmin can, though). What do think
> of that minor tweak?

That's fine from my point of view.

> 
> Is RET_ALLOW allowed to be logged by an application? I guess with it
> default-off for an admin, it should be okay.

As you say, it should be fine to allow but I don't know if there's any
real use in it. I can go either way here.

> 
> Does the app-controlled bitmask apply to the filter, the process, the
> process tree, or something else? e.g. systemd launches an app with a
> filter, leaving the defaults alone, then later process installs a
> filter and wants everything logged -- will things from the earlier
> filter get logged?

I think implementation preferences may decide many of these questions.
As I see it, here are the options in order of my preference:

A) Claim the MSB of the filter return value and make the app logging
   preference per-rule
   - If the bit is set, log the rule
   - Provides very fine-grained logging control at the "high cost" of
     the remaining free bit in the filter return bitmask
   - The bit can be ignored in the case of RET_KILL
   - Can be synced across all threads in the calling task with the
     SECCOMP_FILTER_FLAG_TSYNC filter flag

B) Claim a few bits in the filter flags and make the app logging
   preference per-filter
   - Something like SECCOMP_FILTER_FLAG_LOG_TRAP,
     SECCOMP_FILTER_FLAG_LOG_ERRNO, and
     SECCOMP_FILTER_FLAG_LOG_TRACE
   - Logging for RET_KILL and RET_LOG can't be turned off
   - I'd prefer not to waste a bit for RET_ALLOW in this case so it
     simply won't be loggable
   - Works with the SECCOMP_FILTER_FLAG_TSYNC filter flag
   - Doesn't scale well if many new actions are added in the future

C) A simplified version of 'B' where only a single mode bit is claimed
   to enable logging for all actions except RET_ALLOW
   - Something like SECCOMP_FILTER_FLAG_LOG_ACTIONS
   - Filters without this flag only log RET_KILL and RET_LOG
   - Scales much better than 'B' at the expense of less flexibility
   - Works with the SECCOMP_FILTER_FLAG_TSYNC filter flag

D) Claim a bit in the filter mode and make the app logging preference
   per-process
   - This new SECCOMP_MODE_ENABLE_LOGGING mode would take a bitmask of
     actions that should be logged
   - Incurs a small per-task increase in memory footprint in the form
     of an additional member in 'struct seccomp'
   - Has odd behavior you described above where launchers may set the
     logging preference and then launched application may want
     something different

I think 'A' is the cleanest design but I don't know if highly
configurable logging is deserving of the MSB bit in the filter return.
I'd like to hear your thoughts there.

I _barely_ prefer 'B' over 'C'. They're essential equal in my use case.

To be honest, I haven't completely wrapped my head around how 'D' would
actually work in practice so I may be writing it off prematurely.

Am I missing any more clever options that you can think of? Let me know
what you think of the possibilities.

Tyler

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