Messages in this thread | | | From | "Jason A. Donenfeld" <> | Date | Mon, 26 Sep 2022 14:56:29 +0200 | Subject | Re: [PATCH 2/2] m68k: virt: generate new RNG seed on reboot |
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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.
(A) sounds a lot easier to me. Opinions?
Jason
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