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. >>> 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. It should fail -- we don't want to run the risk of effectively replacing a filter out from under a thread. Adding additional restrictions is safe as long as we retain the nnp from the caller . -Kees -- Kees Cook Chrome OS Security -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
