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

[Laurent Vivier]

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@xxxxxxxxx> 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@xxxxxxxxx> 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@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
> > >
> > > 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.

Thanks,
Laurent
> (A) sounds a lot easier to me. Opinions?
>
> Jason