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

[Kees Cook]

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

-Kees

-- 
Kees Cook
Chrome OS Security
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