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 -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
