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

[Andy Lutomirski]

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.

>
>> If you did that, you'd have to decide whether to continue requiring
>> that all the other threads have a filter that's an ancestor of the
>> current thread's filter.
>
> This is required no matter what to make sure there is no way to
> replace a filter tree with a different one (allowing accidental
> bypasses, misbehavior, etc).

What I mean is:  should the add-new-filter-to-all-threads operation
add the new filter to all threads, regardless of what their current
state is, or should it fail if any thread has a filter that isn't an
ancestor of the current thread's filter?  Either version should be
safe.

--Andy
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