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. > 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). -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
