linux-kernel - Re: Hashed pointer issues

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Date:   Tue, 1 May 2018 07:23:05 +1000
From:   "Tobin C. Harding" <me@...in.cc>
To:     Kees Cook <keescook@...omium.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Ted Ts'o <tytso@....edu>, Steven Rostedt <rostedt@...dmis.org>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Subject: Re: Hashed pointer issues

On Mon, Apr 30, 2018 at 12:16:45PM -0700, Kees Cook wrote:
> On Mon, Apr 30, 2018 at 12:00 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> > On Mon, Apr 30, 2018 at 11:38 AM Kees Cook <keescook@...omium.org> wrote:
> >
> >> Something like this? (Untested.)
> >
> > Looks workable.
> >
> >> +       /* If we have hw RNG, start hashing immediately. */
> >> +       if (arch_has_random()) {
> >> +               get_random_bytes_arch(&ptr_key, sizeof(ptr_key));
> >> +               ptr_key_ready();
> >> +               return 0;
> >> +       }
> >
> > Small tweak: you should check the return value of get_random_bytes_arch(),
> > because in theory it can fail.
> >
> > Sadly, that's not actually how get_random_bytes_arch() really works - it
> > falls back on "get_random_bytes()" on failure instead, which is explicitly
> > against the whole point here.
> 
> I just noticed: there are _no_ users of get_random_bytes_arch() ...
> didn't we once use it to feed entropy to the CRNG?
> 
> > So I think it would need some tweaking, with a new function entirely
> > (get_random_bytes_arch() with a failure return for "cannot fill buffer").
> >
> > But that would be just a few more lines, because we could make the existing
> > get_random_bytes_arch() just use the failure-case thing.
> >
> > So add a "get_hw_random_bytes()" that does that same loop in
> > get_random_bytes_arch(), but returns the number of bytes it filled in.
> >
> > Then get_random_bytes_arch() turns into
> >
> >      got = get_hw_random_bytes(p, nbytes);
> >      if (got < nbytes)
> >          get_random_bytes(p+got, nbytes-got);
> >
> > and the initialize_ptr_random() use would be something like
> >
> >      if (get_hw_random_bytes(&ptr_key, sizeof(ptr_key)) == sizeof(ptr_key)) {
> >          ptr_key_ready();
> >          return 0;
> >     }
> >
> > Hmm?
> >
> > Maybe we could call the "get_hw_random_bytes()" something like
> > "get_early_random_bytes()" and the "use HW for it" is purely an
> > implementation detail?
> 
> Yeah, and if we add __must_check, I think this should be fine. Ted,
> any thoughts on this?
> 
> Tobin, is this something you've got time to implement and test?

Sure thing, thanks for the opportunity.

	Tobin