linux-kernel - Re: [PATCH 2/2] m68k: virt: generate new RNG seed on reboot

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Date:   Mon, 26 Sep 2022 14:02:27 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Laurent Vivier <laurent@...ier.eu>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] m68k: virt: generate new RNG seed on reboot

Hi Laurent,

On Fri, Sep 23, 2022 at 3:10 PM Laurent Vivier <laurent@...ier.eu> wrote:
>
> Le 23/09/2022 à 14:50, Geert Uytterhoeven a écrit :
> > Hi Jason,
> >
> > On Fri, Sep 23, 2022 at 2:26 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
> >> On Fri, Sep 23, 2022 at 2:23 PM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> >>>>>> +       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;
> >>>
> >>> Storing the length should use the proper cpu_to_be16 accessor.
> >>
> >> Okay, I'll do that for v2.
> >>
> >> (Simply out of curiosity, why? Isn't m68k always big endian and this
> >> is arch/ code?)
> >
> > Yes it is.  But virt_parse_bootinfo() below already uses the right
> > accessor.
> >
> > BTW, I guess people thought the same about PowerPC?
> > Although I agree the probability of someone creating a little-endian
> > m68k clone in an FPGA or SkyWater project and trying to run Linux on
> > it quite low ;-)
> >
> >>>> The way I tested this is by having my initramfs just call
> >>>> `reboot(RB_AUTOBOOT);`, and having add_bootloader_randomness() print
> >>>> its contents to the console. I checked that it was both present and
> >>>> different every time.
> >>>
> >>> Are you sure the new kernel did receive the same randomness as prepared
> >>> by get_random_bytes()? I would expect it to just reboot into qemu,
> >>> reload the kernel from disk, and recreate a new bootinfo from scratch,
> >>> including generating a new random seed.
> >>
> >> Yes I'm sure. Without this patch, the new kernel sees the zeroed state.
> >
> > That's interesting.  So QEMU preserves the old bootinfo, which is
> > AFAIK not guaranteed to be still available (that's why I added
> > save_bootinfo()).  Perhaps that works because only memory starting
> > from a rounded-up value of _end will be used, and you're just lucky?
> > I'm wondering what else it preserves. It sure has to reload the
> > kernel image, as at least the data section will no longer contain the
> > initialization values after a reboot...
> >
> > Laurent?
> >
>
> In QEMU the loader makes a copy of the kernel and the initrd and this copy is restored on a reset.
>
> I don't think there is a mechanism in QEMU to save the BOOTINFO section, so I think it works by
> luck. I will check.
>
> Thanks,
> Laurent

Are you sure about that? Or at least, could you point me to where you
think this happens? I'm not as familiar as you with this code base,
but I really am not seeing it. So far as I can tell, on reset, the pc
and stack are reset to their initial places, after TCG resets the cpu
registers to a known state. But the kernel is not reloaded. The same
thing that was in memory before is used again.

Jason