Messages in this thread Patch in this message | | | Date | Mon, 22 Aug 2022 14:16:03 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v1] seccomp: Release filter when copy_process() fails. |
| |
On Mon, Aug 22, 2022 at 01:44:36PM -0700, Kuniyuki Iwashima wrote: > Our syzbot instance reported memory leaks in do_seccomp() [0], similar > to the report [1]. It shows that we miss freeing struct seccomp_filter > and some objects included in it. > > We can reproduce the issue with the program below [2] which calls one > seccomp() and two clone() syscalls. > > The first clone()d child exits earlier than its parent and sends a > signal to kill it during the second clone(), more precisely before the > fatal_signal_pending() test in copy_process(). When the parent receives > the signal, it has to destroy the embryonic process and return -EINTR to > user space. In the failure path, we have to call seccomp_filter_release() > to decrement the filter's ref count. > > Initially, we called it in free_task() called from the failure path, but > the commit 3a15fb6ed92c ("seccomp: release filter after task is fully > dead") moved it to release_task() to notify user space as early as possible > that the filter is no longer used. > > To keep the change, let's call seccomp_filter_release() in copy_process() > and add a WARN_ON_ONCE() in free_task() for future debugging.
Thanks for tracking this down! I think I'd prefer to avoid changing the semantics around the existing seccomp refcount lifetime, so what about just moving copy_seccomp() below the last possible error path?
diff --git a/kernel/fork.c b/kernel/fork.c index 90c85b17bf69..e7f4e7f1e01e 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -2409,12 +2409,6 @@ static __latent_entropy struct task_struct *copy_process( spin_lock(¤t->sighand->siglock); - /* - * Copy seccomp details explicitly here, in case they were changed - * before holding sighand lock. - */ - copy_seccomp(p); - rv_task_fork(p); rseq_fork(p, clone_flags); @@ -2431,6 +2425,14 @@ static __latent_entropy struct task_struct *copy_process( goto bad_fork_cancel_cgroup; } + /* No more failures paths after this point. */ + + /* + * Copy seccomp details explicitly here, in case they were changed + * before holding sighand lock. + */ + copy_seccomp(p); + init_task_pid_links(p); if (likely(p->pid)) { ptrace_init_task(p, (clone_flags & CLONE_PTRACE) || trace);
Totally untested, but I think it would fix this?
-Kees
-- Kees Cook
| |