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

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Date:   Thu, 19 Oct 2017 09:00:44 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     "Tobin C. Harding" <me@...in.cc>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        kernel-hardening@...ts.openwall.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Tycho Andersen <tycho@...ker.com>,
        "Roberts, William C" <william.c.roberts@...el.com>,
        Tejun Heo <tj@...nel.org>,
        Jordan Glover <Golden_Miller83@...tonmail.ch>,
        Greg KH <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>, Joe Perches <joe@...ches.com>,
        Ian Campbell <ijc@...lion.org.uk>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Fries <cfries@...gle.com>,
        Dave Weinstein <olorin@...gle.com>,
        Daniel Micay <danielmicay@...il.com>,
        Djalal Harouni <tixxdz@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] printk: hash addresses printed with %p

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