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

[Kees Cook]

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.

> 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).

-Kees

-- 
Kees Cook
Chrome OS Security
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