Re: [PATCH v5 6/6] seccomp: add SECCOMP_EXT_ACT_TSYNC and SECCOMP_FILTER_TSYNC

[Andy Lutomirski]

On Tue, May 27, 2014 at 11:45 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> On Tue, May 27, 2014 at 11:40 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Tue, May 27, 2014 at 11:24 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>> On Mon, May 26, 2014 at 12:27 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>> On Fri, May 23, 2014 at 10:05 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>>> On Thu, May 22, 2014 at 4:11 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>>>> On Thu, May 22, 2014 at 4:05 PM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>>>>> Applying restrictive seccomp filter programs to large or diverse
>>>>>>> codebases often requires handling threads which may be started early in
>>>>>>> the process lifetime (e.g., by code that is linked in). While it is
>>>>>>> possible to apply permissive programs prior to process start up, it is
>>>>>>> difficult to further restrict the kernel ABI to those threads after that
>>>>>>> point.
>>>>>>>
>>>>>>> This change adds a new seccomp extension action for synchronizing thread
>>>>>>> group seccomp filters and a prctl() for accessing that functionality,
>>>>>>> as well as a flag for SECCOMP_EXT_ACT_FILTER to perform sync at filter
>>>>>>> installation time.
>>>>>>>
>>>>>>> When calling prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT, SECCOMP_EXT_ACT_FILTER,
>>>>>>> flags, filter) with flags containing SECCOMP_FILTER_TSYNC, or when calling
>>>>>>> prctl(PR_SECCOMP_EXT, SECCOMP_EXT_ACT, SECCOMP_EXT_ACT_TSYNC, 0, 0), it
>>>>>>> will attempt to synchronize all threads in current's threadgroup to its
>>>>>>> seccomp filter program. This is possible iff all threads are using a filter
>>>>>>> that is an ancestor to the filter current is attempting to synchronize to.
>>>>>>> NULL filters (where the task is running as SECCOMP_MODE_NONE) are also
>>>>>>> treated as ancestors allowing threads to be transitioned into
>>>>>>> SECCOMP_MODE_FILTER. If prctrl(PR_SET_NO_NEW_PRIVS, ...) has been set on the
>>>>>>> calling thread, no_new_privs will be set for all synchronized threads too.
>>>>>>> On success, 0 is returned. On failure, the pid of one of the failing threads
>>>>>>> will be returned, with as many filters installed as possible.
>>>>>>
>>>>>> Is there a use case for adding a filter and synchronizing filters
>>>>>> being separate operations?  If not, I think this would be easier to
>>>>>> understand and to use if there was just a single operation.
>>>>>
>>>>> Yes: if the other thread's lifetime is not well controlled, it's good
>>>>> to be able to have a distinct interface to retry the thread sync that
>>>>> doesn't require adding "no-op" filters.
>>>>
>>>> Wouldn't this still be solved by:
>>>>
>>>> seccomp_add_filter(final_filter, SECCOMP_FILTER_ALL_THREADS);
>>>>
>>>> the idea would be that, if seccomp_add_filter fails, then you give up
>>>> and, if it succeeds, then you're done.  It shouldn't fail unless out
>>>> of memory or you've nested too deeply.
>>>
>>> I wanted to keep the case of being able to to wait for non-ancestor
>>> threads to finish. For example, 2 threads start and set separate
>>> filters. 1 does work and exits, 2 starts another thread (3) which adds
>>> filters, does work, and then waits for 1 to finish by calling TSYNC.
>>> Once 1 dies, TSYNC succeeds. In the case of not having direct control
>>> over thread lifetime (say, when using third-party libraries), I'd like
>>> to retain the flexibility of being able to do TSYNC without needing a
>>> filter being attached to it.
>>
>> I must admit this strikes me as odd.  What's the point of having a
>> thread set a filter if it intends to be a short-lived thread?
>
> I was illustrating the potential insanity of third-party libraries.
> There isn't much sense in that behavior, but if it exists, working
> around it is harder without the separate TSYNC-only call.
>
>> In any case, I must have missed the ability for TSYNC to block.  Hmm.
>> That seems complicated, albeit potentially useful.
>
> Oh, no, I didn't mean to imply TSYNC should block. I meant that thread
> 3 could do:
>
> while (TSYNC-fails)
>    wait-on-or-kill-unexpected-thread
>

Ok.

I'm still not seeing the need for a separate TSYNC option, though --
just add-a-filter-across-all-threads would work if it failed
harmlessly on error.  FWIW, TSYNC is probably equivalent to adding an
always-accept filter across all threads, although no one should really
do the latter for efficiency reasons.

--Andy

> -Kees
>
> --
> Kees Cook
> Chrome OS Security



-- 
Andy Lutomirski
AMA Capital Management, LLC
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