linux-kernel - Re: Hashed pointer issues

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Date:   Mon, 30 Apr 2018 12:16:45 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Tobin C. Harding" <me@...in.cc>, "Ted Ts'o" <tytso@....edu>
Cc:     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 12:00 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?

-Kees

-- 
Kees Cook
Pixel Security