[PATCH 3/5] v2 seccomp_filters: Enable ftrace-based system call filtering

[Will Drewry]

On Sat, May 14, 2011 at 2:30 AM, Ingo Molnar <mingo at elte.hu> wrote:
>
> * Eric Paris <eparis at redhat.com> wrote:
>
>> [dropping microblaze and roland]
>>
>> lOn Fri, 2011-05-13 at 14:10 +0200, Ingo Molnar wrote:
>> > * James Morris <jmorris at namei.org> wrote:
>>
>> > It is a simple and sensible security feature, agreed? It allows most code to
>> > run well and link to countless libraries - but no access to other files is
>> > allowed.
>>
>> It's simple enough and sounds reasonable, but you can read all the discussion
>> about AppArmour why many people don't really think it's the best. [...]
>
> I have to say most of the participants of the AppArmour flamefests were dead
> wrong, and it wasnt the AppArmour folks who were wrong ...
>
> The straight ASCII VFS namespace *makes sense*, and yes, the raw physical
> objects space that SELinux uses makes sense as well.
>
> And no, i do not subscribe to the dogma that it is not possible to secure the
> ASCII VFS namespace: it evidently is possible, if you know and handle the
> ambiguitites. It is also obviously true that the ASCII VFS namespaces we use
> every day are a *lot* more intuitive than the labeled physical objects space
> ...
>
> What all the security flamewars missed is the simple fact that being intuitive
> matters a *lot* not just to not annoy users, but also to broaden the effective
> number of security-conscious developers ...
>
>> > Unfortunately this audit callback cannot be used for my purposes, because
>> > the event is single-purpose for auditd and because it allows no feedback
>> > (no deny/accept discretion for the security policy).
>> >
>> > But if had this simple event there:
>> >
>> >     err = event_vfs_getname(result);
>>
>> Wow it sounds so easy.  Now lets keep extending your train of thought
>> until we can actually provide the security provided by SELinux.  What do
>> we end up with?  We end up with an event hook right next to every LSM
>> hook.  You know, the LSM hooks were placed where they are for a reason.
>> Because those were the locations inside the kernel where you actually
>> have information about the task doing an operation and the objects
>> (files, sockets, directories, other tasks, etc) they are doing an
>> operation on.
>>
>> Honestly all you are talking about it remaking the LSM with 2 sets of
>> hooks instead if 1.  Why? [...]
>
> Not at all. I am taking about using *one* set of events, to keep the intrusion
> at the lowest possible level.
>
> LSM could make use of them as well.
>
> Obviously for pragmatic reasons that might not be feasible initially.
>
>> [...]  It seems much easier that if you want the language of the filter
>> engine you would just make a new LSM that uses the filter engine for it's
>> policy language rather than the language created by SELinux or SMACK or name
>> your LSM implementation.
>
> Correct, that is what i suggested.
>
> Note that performance is a primary concern, so if certain filters are very
> popular then in practice this would come with support for a couple of 'built
> in' (pre-optimized) filters that the kernel can accelerate directly, so that we
> do not incure the cost of executing the filter preds for really common-sense
> security policies that almost everyone is using.
>
> I.e. in the end we'd *roughly* end up with the same performance and security as
> we are today (i mean, SELinux and the other LSMs did a nice job of collecting
> the things that apps should be careful about), but the crutial difference isnt
> just the advantages i menioned, but the fact that the *development model* of
> security modules would be a *lot* more extensible.
>
> So security efforts could move to a whole different level: they could move into
> key apps and they could integrate with the general mind-set of developers.
>
> At least Will as an application framework developer cares, so that hope is
> justified i think.
>
>> >  - unprivileged:  application-definable, allowing the embedding of security
>> >                   policy in *apps* as well, not just the system
>> >
>> >  - flexible:      can be added/removed runtime unprivileged, and cheaply so
>> >
>> >  - transparent:   does not impact executing code that meets the policy
>> >
>> >  - nestable:      it is inherited by child tasks and is fundamentally stackable,
>> >                   multiple policies will have the combined effect and they
>> >                   are transparent to each other. So if a child task within a
>> >                   sandbox adds *more* checks then those add to the already
>> >                   existing set of checks. We only narrow permissions, never
>> >                   extend them.
>> >
>> >  - generic:       allowing observation and (safe) control of security relevant
>> >                   parameters not just at the system call boundary but at other
>> >                   relevant places of kernel execution as well: which
>> >                   points/callbacks could also be used for other types of event
>> >                   extraction such as perf. It could even be shared with audit ...
>>
>> I'm not arguing that any of these things are bad things.  What you describe
>> is a new LSM that uses a discretionary access control model but with the
>> granularity and flexibility that has traditionally only existed in the
>> mandatory access control security modules previously implemented in the
>> kernel.
>>
>> I won't argue that's a bad idea, there's no reason in my mind that a process
>> shouldn't be allowed to control it's own access decisions in a more flexible
>> way than rwx bits.  Then again, I certainly don't see a reason that this
>> syscall hardening patch should be held up while a whole new concept in
>> computer security is contemplated...
>
> Note, i'm not actually asking for the moon, a pony and more.
>
> I fully submit that we are yet far away from being able to do a full LSM via
> this mechanism.
>
> What i'm asking for is that because the syscall point steps taken by Will look
> very promising so it would be nice to do *that* in a slightly more flexible
> scheme that does not condemn it to be limited to the syscall boundary and such
> ...

What do you suggest here?

>From my brief exploration of the ftrace/perf (and seccomp) code, I
don't see a clean way of integrating over the existing interfaces to
the ftrace framework (e.g., the global perf event pump seems to be a
mismatch), but I may be missing something obvious.   In my view,
implementing this nestled between the seccomp/ftrace world provides a
stepping stone forward without being too restrictive.  No matter how
we change security events in the future, system calls will always be
the first line of attack surface reduction.  It could just mean that,
in the long term, accessing the "security event filtering" framework
is done through another new interface with seccomp providing only a
targeted syscall filtering featureset that may one day be deprecated
(if that day ever comes).

If there's a clear way to cleanly expand this interface that I'm
missing, I'd love to know - thanks!
will

> Also, to answer you, do you say that by my argument someone should have stood
> up and said 'no' to the LSM mess that was introduced a couple of years ago and
> which caused so many problems:
>
>  - kernel inefficiencies and user-space overhead
>
>  - stalled security efforts
>
>  - infighting
>
>  - friction, fragmentation, overmodularization
>
>  - non-stackability
>
>  - security annoyances on the Linux desktop
>
>  - probably *less* Linux security
>
> and should have asked them to do something better designed instead?
>
> Thanks,
>
>        Ingo
>