Re: [PATCH 1/2] seccomp: don't leak memory when filter install races

[Tycho Andersen <tycho@tycho.pizza>]

On Wed, Sep 02, 2020 at 11:08:49AM +0200, Christian Brauner wrote:
> On Tue, Sep 01, 2020 at 07:40:16PM -0600, Tycho Andersen wrote:
> > In seccomp_set_mode_filter() with TSYNC | NEW_LISTENER, we first initialize
> > the listener fd, then check to see if we can actually use it later in
> > seccomp_may_assign_mode(), which can fail if anyone else in our thread
> > group has installed a filter and caused some divergence. If we can't, we
> > partially clean up the newly allocated file: we put the fd, put the file,
> > but don't actually clean up the *memory* that was allocated at
> > filter->notif. Let's clean that up too.
> > 
> > To accomplish this, let's hoist the actual "detach a notifier from a
> > filter" code to its own helper out of seccomp_notify_release(), so that in
> > case anyone adds stuff to init_listener(), they only have to add the
> > cleanup code in one spot. This does a bit of extra locking and such on the
> > failure path when the filter is not attached, but it's a slow failure path
> > anyway.
> > 
> > Fixes: 51891498f2da ("seccomp: allow TSYNC and USER_NOTIF together")
> > Reported-by: syzbot+3ad9614a12f80994c32e@syzkaller.appspotmail.com
> > Signed-off-by: Tycho Andersen <tycho@tycho.pizza>
> > ---
> 
> This looks sane to me!
> Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
> 
> One thing I noticed when checking the failure paths. In init_listener we
> allocate the notifier by directly storing it into filter->notif and if
> anon_inode_getfile() fails we simply kfree(filter->notif) but don't NULL
> it and leave filter->notif pointing to freed memory.
> Since we have a few places where we check filter->notif whether it is
> initialized or not maybe we should NULL filter->notif if init_listener()
> fails or alloc the notifier into a tmp variable and only assign it to
> filter->notif after we can't fail anymore in init_listener().
> 
> Just a thought since the error path in seccomp_set_mode_filter() is a
> little more complex. :)

Yeah it seems like we definitely should, and maybe this is what Kees
was talking about and I missed it.

I'll send a patch on top of this shortly.

Tycho