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. --Andy > -Kees > > -- > Kees Cook > Chrome OS Security -- Andy Lutomirski AMA Capital Management, LLC -- 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
