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

Kees Cook
2017-04-07T22:46:38Z

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-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org> 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:
>
> - 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?

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

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?

-Kees

--
Kees Cook
Pixel Security