Messages in this thread | | | Date | Mon, 26 Sep 2022 15:02:42 +0200 | Subject | Re: [PATCH 2/2] m68k: virt: generate new RNG seed on reboot | From | Laurent Vivier <> |
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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.
Thanks, Laurent > (A) sounds a lot easier to me. Opinions? > > Jason
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