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. > >> 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. --Andy -- 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
