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

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Date:   Sun, 29 Sep 2019 19:59:19 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Ahmed S. Darwish" <darwish.07@...il.com>
Cc:     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 6:16 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But I've committed that patch and the revert of the ext4 revert to a
> local branch, I'll do some basic testing of it (which honestly on my
> machines are kind of pointless, since all of them support rdrand), but
> assuming it passes the basic smoke tests - and I expect it to - I'll
> merge it for rc1.

All my smoke testing looked fine - I disabled trusting the CPU, I
increased the required entropy a lot, and to actually trigger the
lockup issue without the broken user space, I made /dev/urandom do
that "wait for entropy" thing too.

It all looked sane to me, and the urandom part also had the side
effect of then silencing all the "reading urandom without entropy"
warning cases as expected.

So it's merged.

Note that what I merged did _not_ contain the urandom changes, that
was purely for my testing. But it might well be a reasonable thing to
do at some point.

Of course, whether this jitter-entropy approach is reasonable in the
first place ends up likely being debated, but it does seem to be the
simplest way forward.

           Linus