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SubjectRe: [PATCH v1] seccomp: Release filter when copy_process() fails.
Date
From:   Kees Cook <keescook@chromium.org>
Date: Mon, 22 Aug 2022 16:38:07 -0700
> On Mon, Aug 22, 2022 at 02:49:35PM -0700, Kuniyuki Iwashima wrote:
> > 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?
>
> There's no allocation happening in copy_seccomp(), just reference
> counts being added. Given the lock that is held, the ordering here
> doesn't matter as far as I can tell, except for the fact that
> copy_seccomp() expects to go through full thread death if something goes
> wrong. So, simply moving it later should do the trick here.

Ok, I'm fine with that change.
I'll test it again and post v2 with WARN_ON_ONCE().
Thank you!


>
> -Kees
>
> >
> > >
> > >
> > > 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(&current->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
>
> --
> Kees Cook

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