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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Fri, 23 Sep 2022 13:30:06 +0200
Message-ID: <CAMuHMdUuM85s1APoxRmXnw13hHHEGgo8Z9EKvpV6maaZPaVUfA@mail.gmail.com>
Subject: Re: [PATCH 2/2] m68k: virt: generate new RNG seed on reboot
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     linux-m68k@lists.linux-m68k.org, linux-kernel@vger.kernel.org
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Hi Jason,

On Wed, Sep 21, 2022 at 8:52 PM Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> Rather than rebooting into a system with no entropy, regenerate the RNG
> seed before rebooting, so that the new system has a fresh seed.
>
> Fixes: a1ee38ab1a75 ("m68k: virt: Use RNG seed from bootinfo block")
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

Thanks for your patch!

> --- a/arch/m68k/virt/config.c
> +++ b/arch/m68k/virt/config.c
> @@ -45,10 +45,18 @@ static void virt_halt(void)
>                 ;
>  }
>
> +static struct bi_record *rng_seed_record;

This can be const...

> +
>  static void virt_reset(void)
>  {
>         void __iomem *base = (void __iomem *)virt_bi_data.ctrl.mmio;
>
> +       if (rng_seed_record && rng_seed_record->size > sizeof(*rng_seed_record) + 2) {
> +               u16 len = rng_seed_record->size - sizeof(*rng_seed_record) - 2;
> +               get_random_bytes((u8 *)rng_seed_record->data + 2, len);
> +               *(u16 *)rng_seed_record->data = len;

Wouldn't it be simpler to just use the existing length?

    if (rnd_seed_record) {
           u16 len = be16_to_cpup(data);
           get_random_bytes((u8 *)rng_seed_record->data + 2, len);
    }

However, I have my doubts this will actually work. Was this tested?
The bootinfo is passed from userspace, usually by reading
/proc/bootinfo, and adapting it where needed.
So I think you should implement this in kexec-tools instead.

> +       }
> +
>         iowrite32be(CMD_RESET, base + VIRT_CTRL_REG_CMD);
>         local_irq_disable();
>         while (1)
> @@ -101,6 +109,8 @@ int __init virt_parse_bootinfo(const struct bi_record *record)
>                  * length to prevent kexec from using it.
>                  */
>                 memzero_explicit((void *)data, len + 2);
> +                /* Store a reference to be filled in on reboot. */
> +               rng_seed_record = (void *)record;

... so this cast can be dropped.

>                 break;
>         }
>         default:

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds