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

[Kees Cook]

On Tue, May 27, 2014 at 12:10 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> 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.

Given the complexity of the locking, "fail" means "I applied the
change to all threads except for at least this one: *error code*",
which means looping with the "add-a-filter" method means all the other
threads keep getting filters added until there is full success. I
don't want that overhead, so this keeps TSYNC distinctly separate.

Because of the filter addition, when using add_filter-TSYNC, it's not
sensible to continue after a failure. However, using just-TSYNC allows
sensible re-trying. Since the environments where TSYNC intend to be
used in can be very weird, I really want to retain the retry ability.

-Kees

-- 
Kees Cook
Chrome OS Security
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/