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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

On Mon, Sep 26, 2022 at 3:02 PM Laurent Vivier <laurent@vivier.eu> wrote:
>
> Le 26/09/2022 à 14:56, Jason A. Donenfeld a écrit :
> > Hi Laurent,
> >
> > On Mon, Sep 26, 2022 at 2:52 PM Laurent Vivier <laurent@vivier.eu> wrote:
> >>
> >> Hi Jason,
> >>
> >> Le 26/09/2022 à 14:02, Jason A. Donenfeld a écrit :
> >>> 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.
> >>
> >> Yes, this is not clear in QEMU but I think this happens in rom_reset():
> >>
> >> hw/core/loader.c
> >>
> >> 1180         if (rom->mr) {
> >> 1181             void *host = memory_region_get_ram_ptr(rom->mr);
> >> 1182             memcpy(host, rom->data, rom->datasize);
> >> 1183             memset(host + rom->datasize, 0, rom->romsize - rom->datasize);
> >> 1184         } else {
> >> 1185             address_space_write_rom(rom->as, rom->addr, MEMTXATTRS_UNSPECIFIED,
> >> 1186                                     rom->data, rom->datasize);
> >> 1187             address_space_set(rom->as, rom->addr + rom->datasize, 0,
> >> 1188                               rom->romsize - rom->datasize,
> >> 1189                               MEMTXATTRS_UNSPECIFIED);
> >> 1190         }
> >>
> >> kernel and initrd are loaded with load_elf() and load_image_targphys() only once at startup by the
> >> machine init function (virt_init()).
> >>
> >> rom_add_elf_program() adds the kernel to the ROM list
> >> (in include/hw/elf_ops.h, glue(load_elf, SZ) that generates load_elf32() when SZ is 32...)
> >>
> >> rom_add_file() adds the initrd to the ROM list too.
> >>
> >> And ROMs are restored on reset from these copies by rom_reset().
> >>
> >> rom_reset() is registered as a reset handler with qemu_register_reset() by
> >> rom_check_and_register_reset() at the end of the machine creation by qdev_machine_creation_done().
> >>
> >> So I think bootinfo are not restored because there is no such function calls. Perhaps they are saved
> >> and restaured if they are stored in address space of one of the previous registered ROM.
> >
> > Ahh interesting, thanks for the explanation.
> >
> > So from my debugging, bootinfo is *not* restored, and the previous one
> > appears to be used. Fortunately it's intact and everything works well
> > on a reboot.
> >
> > With that in mind, we now we have to decide whether to:
> > A) Go with my linux patch to write the rng seed before rebooting (3/3
> > in v4 of that series).
> > B) Not go with the linux patch, but instead make sure bootinfo is
> > restored to its previous value, and then also register a qemu reboot
> > notifier to refresh the seed in it, like what x86 does.
> >
>
> I prefer B :)
> It's cleaner and under QEMU control.

Okay. I'm happy to follow your preference. Just one last question,
though: is this what happens on baremetal bootloaders too? Or does no
such thing really exist so it doesn't matter?

Jason