On Fri, May 29, 2020 at 1:11 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > On Thu, May 28, 2020 at 05:14:11PM +0200, Christian Brauner wrote: > > * @usage: reference count to manage the object lifetime. > > * get/put helpers should be used when accessing an instance > > * outside of a lifetime-guarded section. In general, this > > * is only needed for handling filters shared across tasks. > > [...] > > + * @live: Number of tasks that use this filter directly and number > > + * of dependent filters that have a non-zero @live counter. > > + * Altered during fork(), exit(), and filter installation > > [...] > > refcount_set(&sfilter->usage, 1); > > + refcount_set(&sfilter->live, 1); [...] > After looking at these other lifetime management examples in the kernel, > I'm convinced that tracking these states separately is correct, but I > remain uncomfortable about task management needing to explicitly make > two calls to let go of the filter. > > I wonder if release_task() should also detach the filter from the task > and do a put_seccomp_filter() instead of waiting for task_free(). This > is supported by the other place where seccomp_filter_release() is > called: > > > @@ -396,6 +400,7 @@ static inline void seccomp_sync_threads(unsigned long flags) > > * allows a put before the assignment.) > > */ > > put_seccomp_filter(thread); > > + seccomp_filter_release(thread); > > This would also remove the only put_seccomp_filter() call outside of > seccomp.c, since the free_task() call will be removed now in favor of > the task_release() call. > > So, is it safe to detach the filter in release_task()? Has dethreading > happened yet? i.e. can we race TSYNC? -- is there a possible > inc-from-zero? release_task -> __exit_signal -> __unhash_process -> list_del_rcu(&p->thread_node) drops us from the thread list under siglock, which is the same lock TSYNC uses. One other interesting thing that can look at seccomp state is task_seccomp() in procfs - that can still happen at this point. At the moment, procfs only lets you see the numeric filter state, not the actual filter contents, so that's not a problem; but if we ever add a procfs interface for dumping seccomp filters (in addition to the ptrace interface that already exists), that's something to keep in mind. > (Actually, all our refcount_inc()s should be > refcount_inc_not_zero() just for robustness.) Eeeh... wouldn't that just make the code more complicated for no good reason? > I *think* we can do it > before the release_thread() call (instead of after cgroup_release()). _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
