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

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Date:   Tue, 1 Oct 2019 10:14:40 -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 Tue, Oct 1, 2019 at 6:51 AM Borislav Petkov <bp@...en8.de> wrote:
>
> So when I add this by hand and do git diff, it adds a second hunk:

You and me both.

We both have editors that always add line-endings, and right now that
file doesn't have a newline at the end of the file thanks to commit
428826f5358c ("fdt: add support for rng-seed").


> and I kinda get what it is trying to tell me but this is new. And when I
> do
>
> $ xxd drivers/char/random.c
> ..
>
> 000125e0: 646f 6d6e 6573 7329 3b0a                 domness);.
>
> there's a 0xa at the end so what's git really trying to tell me?

The previous state of the file didn't have that 0xa at the end, so you get that


  -EXPORT_SYMBOL_GPL(add_bootloader_randomness);
  \ No newline at end of file
  +EXPORT_SYMBOL_GPL(add_bootloader_randomness);

which is "the '-' line doesn't have a newline, the '+' line does" marker.

> > 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.
>
> So make it a separate patch and let's hammer on it during the next weeks
> and see what happens?

Yeah, probably.

> > 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
>
> Wouldn't that even increase its entropy, which would be a good thing?

I'm not sure it increases it, but it certainly shouldn't make it any worse.

                  Linus