linux-kernel - Re: x86/random: Speculation to the rescue

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Date:   Mon, 30 Sep 2019 09:06:36 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Nicholas Mc Guire <hofrat@...ntech.at>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: x86/random: Speculation to the rescue

On Sun, Sep 29, 2019 at 11:10 PM Borislav Petkov <bp@...en8.de> wrote:
>
> so sshd does getrandom(2) while those other userspace things don't. Oh
> well.

Well, that's actually what systems _should_ do. Presumably sshd
actually wants secure randomness, and nothing else waits for it.

Obviously, that can be a problem if you then need sshd in order to get
into a headless box, so my patch fixes things for you too, but at
least your box doesn't show the problem that Ahmed had, and the boot
completing presumably means that you got more entropy from other disk
IO being done by the rest of the boot.

If you want to test my hacky "do /dev/urandom too", it was this one-liner:

  --- a/drivers/char/random.c
  +++ b/drivers/char/random.c
  @@ -2027,6 +2027,7 @@ urandom_read(struct file *file, char __user
*buf, size_t nbytes, loff_t *ppos)
        static int maxwarn = 10;
        int ret;

  +     if (!crng_ready()) try_to_generate_entropy();
        if (!crng_ready() && maxwarn > 0) {
                maxwarn--;
                if (__ratelimit(&urandom_warning))

and that should get rid of the warnings.

It's not using the full "wait_for_random_bytes()", because in the
absence of a cycle counter, that would introduce the boot-time lockup
for /dev/urandom too.

Doing something like the above to /dev/urandom is likely the right
thing to do eventually, but I didn't want to mix up "we can perhaps
improve the urandom situation too" with the basic "let's fix the boot
problem". The urandom behavior change would be a separate thing.

Also, talking about "future changes". Right now
"try_to_generate_entropy()" is actually uninterruptible once it gets
started. I think we should add a test for signal_pending() too, but it
should generally complete really fairly quickly so I left it without
one just to see if anybody even notices.

                 Linus