Messages in this thread | | | From | Kuniyuki Iwashima <> | Subject | Re: [PATCH v1] seccomp: Release filter when copy_process() fails. | Date | Mon, 22 Aug 2022 14:49:35 -0700 |
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From: Kees Cook <keescook@chromium.org> Date: Mon, 22 Aug 2022 14:16:03 -0700 > 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?
Actually, I also thought of it but avoid it because it means we move the signal check relatively earlier than before, so would-be-killed processes could consume more resouces.
What do you think about this?
> > > 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
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