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

Steve Grubb
2017-04-10T15:18:21Z

On Friday, April 7, 2017 6:16:08 PM EDT Tyler Hicks 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@can
> >>>> onical.com v1:
> >>>> http://lkml.kernel.org/r/1483377999-15019-2-git-send-email-tyhicks@can
> >>>> onical.com
> >>>>
> >>>> v2:
> >>>> http://lkml.kernel.org/r/1486100262-32391-1-git-send-email-tyhicks@can
> >>>> onical.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.

Just to illustrate what can happen, /usr/lib64/qt5/libexec/QtWebEngineProcess
is causing hundreds of errno SECCOMP audit events.

See bz https://bugzilla.redhat.com/show_bug.cgi?id=1438973

The developers should not have enabled this for distribution's runtime. It
should be a debug option.

-Steve

> 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.
>
> Tyler