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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Kees Cook <keescook@chromium.org>
Cc: tcharding <me@tobin.cc>, Steven Rostedt <rostedt@goodmis.org>,
	Anna-Maria Gleixner <anna-maria@linutronix.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: Hashed pointer issues
Date: Mon, 30 Apr 2018 19:00:49 +0000
Message-ID: <CA+55aFyreOY8uzOkkDkU=WttNfbH2_6nStfsm9j2T6F5ZyntHg@mail.gmail.com> (raw)
In-Reply-To: <CAGXu5jLVJxDJcbhrQU5-wduaP+OvePXP8VvCFMrJ47pYD-zaXg@mail.gmail.com>

On Mon, Apr 30, 2018 at 11:38 AM Kees Cook <keescook@chromium.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.

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?

                Linus

  reply index

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-30 15:50 Anna-Maria Gleixner
2018-04-30 16:11 ` Kees Cook
2018-04-30 16:31   ` Linus Torvalds
2018-04-30 16:41     ` Steven Rostedt
2018-04-30 16:57       ` Linus Torvalds
2018-04-30 17:01         ` Linus Torvalds
2018-04-30 17:06           ` Randy Dunlap
2018-04-30 17:12             ` Linus Torvalds
2018-05-01  7:05               ` tcharding
2018-05-03  8:41             ` Geert Uytterhoeven
2018-04-30 18:38           ` Kees Cook
2018-04-30 19:00             ` Linus Torvalds [this message]
2018-04-30 19:16               ` Kees Cook
2018-04-30 20:01                 ` Linus Torvalds
2018-04-30 20:07                   ` Linus Torvalds
2018-04-30 21:23                 ` Tobin C. Harding

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