[kernel-hardening] Re: [PATCH v4] printk: hash addresses printed with %p

[Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>]

On (10/18/17 17:04), Tobin C. Harding wrote:
[..]
> > > +/* protects ptr_secret and have_key */
> > > +DEFINE_SPINLOCK(key_lock);
> > > +static siphash_key_t ptr_secret __read_mostly;
> > > +static atomic_t have_key = ATOMIC_INIT(0);
> > > +
> > > +static int initialize_ptr_secret(void)
> > > +{
> > > +	spin_lock(&key_lock);
> > > +	if (atomic_read(&have_key) == 1)
> > > +		goto unlock;
> > > +
> > > +	get_random_bytes(&ptr_secret, sizeof(ptr_secret));
> > > +	atomic_set(&have_key, 1);
> > > +
> > > +unlock:
> > > +	spin_unlock(&key_lock);
> > > +	return 0;
> > > +}
> > 
> > is this spinlock legal? what happens if we are getting interrupted by NMI?
> 
> I think we can do without the spinlock. I think I was already told that when
> I tried to put it [some where else] in v1.
> 
> It's fun failing in public ;)

another note is that printk()->vscnprintf()->get_random_bytes()->warn_unseeded_randomness()
causes a printk() recursion, but we should be fine now, we are in printk_safe
mode by the time we vscnprintf().

but a bigger problem might the following thing:

vscnprintf()
 pointer()
  ptr_to_id()
   initialize_ptr_secret()
    get_random_bytes()
     _get_random_bytes()
      extract_crng()
       _extract_crng()
        spin_lock_irqsave(&crng->lock, flags);   <<<<<


this, once again, can deadlock. can it? just like before:

> > printk()
> >  vprintk_emit()
> >   vscnprintf()
> >    pointer()
> >     ptr_to_id()
> >      initialize_ptr_secret()
> >       spin_lock(&key_lock)
> > 
> > ----> NMI
> > 
> >       printk()
> >        printk_safe_log_store()
> >         vscnprintf()
> >          pointer()
> >           ptr_to_id()
> >            initialize_ptr_secret()
> >             spin_lock(&key_lock)   <<<<

	-ss