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-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
