linux-kernel - Re: [PATCH v1] seccomp: Release filter when copy

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Message-ID: <202208221636.1DE3D674@keescook>
Date:   Mon, 22 Aug 2022 16:38:07 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     ayudutta@...zon.com, brauner@...nel.org, kuni1840@...il.com,
        linux-kernel@...r.kernel.org, luto@...capital.net,
        syzbot+ab17848fe269b573eb71@...kaller.appspotmail.com,
        wad@...omium.org
Subject: Re: [PATCH v1] seccomp: Release filter when copy_process() fails.

On Mon, Aug 22, 2022 at 02:49:35PM -0700, Kuniyuki Iwashima wrote:
> From:   Kees Cook <keescook@...omium.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.

-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