Messages in this thread | | | From | "Jason A. Donenfeld" <> | Date | Mon, 26 Sep 2022 14:02:27 +0200 | 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@vivier.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@zx2c4.com> wrote: > >> On Fri, Sep 23, 2022 at 2:23 PM Geert Uytterhoeven <geert@linux-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
| |