Re: [PATCH v1] seccomp: Release filter when copy_process() fails.

[Kuniyuki Iwashima <kuniyu@amazon.com>]

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