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

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Message-ID: <eb798159-c003-3b43-c891-039080e06e03@vivier.eu>
Date:   Fri, 23 Sep 2022 15:10:49 +0200
From:   Laurent Vivier <laurent@...ier.eu>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-m68k@...ts.linux-m68k.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] m68k: virt: generate new RNG seed on reboot

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