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

[Laurent Vivier]

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@xxxxxxxxx> wrote:
> > On Fri, Sep 23, 2022 at 2:23 PM Geert Uytterhoeven <geert@xxxxxxxxxxxxxx> 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