[PATCH kexec-tools] m68k: pass rng seed via BI_VIRT_RNG_SEED

[Geert Uytterhoeven]

Hi Jason,

On Fri, Sep 23, 2022 at 2:47 PM Jason A. Donenfeld <Jason at zx2c4.com> wrote:
> In order to pass fresh entropy to kexec'd kernels, use BI_VIRT_RNG_SEED
> for passing a seed, with the same semantics that kexec-tools currently
> uses for i386's setup_data.
>
> Cc: Geert Uytterhoeven <geert at linux-m68k.org>
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>

Thanks for your patch!

> --- a/kexec/arch/m68k/bootinfo.c
> +++ b/kexec/arch/m68k/bootinfo.c
> @@ -135,40 +135,45 @@ void bootinfo_print(void)
>                 size = bi->size;
>                 switch (tag) {
>                 case BI_MACHTYPE:
>                         printf("BI_MACHTYPE: 0x%08x\n", bi->machtype);
>                         break;
>
>                 case BI_MEMCHUNK:
>                         printf("BI_MEMCHUNK: 0x%08x bytes at 0x%08x\n",
>                                bi->mem_info.size, bi->mem_info.addr);
>                         break;
>
>                 case BI_RAMDISK:
>                         printf("BI_RAMDISK: 0x%08x bytes at 0x%08x\n",
>                                bi->mem_info.size, bi->mem_info.addr);
>                         break;

Hmm, lots of context.

>
>                 case BI_COMMAND_LINE:
>                         printf("BI_COMMAND_LINE: %s\n", bi->string);
>                         break;
>
> +               case BI_VIRT_RNG_SEED:

This won't work as expected: BI_VIRT_* tags are only valid if the
machine type is MACH_VIRT.

Worse, this will trigger for other platform-specific tags using the same value:

    arch/m68k/include/uapi/asm/bootinfo-amiga.h:#define
BI_AMIGA_CHIPSET    0x8006  /* native chipset present (__be32) */
    arch/m68k/include/uapi/asm/bootinfo-mac.h:#define BI_MAC_SCCBASE
             0x8006  /* Mac SCC base address */
    arch/m68k/include/uapi/asm/bootinfo-virt.h:#define
BI_VIRT_RNG_SEED     0x8006

> +                       /* These are secret, so never print them to the console */
> +                       printf("BI_VIRT_RNG_SEED: 0x%08x bytes\n", be16_to_cpu(bi->bytes.len));
> +                       break;
> +
>                 default:
>                         printf("BI tag 0x%04x size %u\n", tag, size);
>                         break;
>                 }
>                 bi = bi_next(bi, size);
>         }
>  }

>  }
>
> +void bootinfo_add_rng_seed(void)
> +{
> +       struct bi_rec *bi;
> +       enum { RNG_SEED_LEN = 32 };
> +
> +       /* Remove existing rng seed records */
> +       bi_remove(BI_VIRT_RNG_SEED);
> +
> +       /* Add new rng seed record */
> +       bi = bi_add(BI_VIRT_RNG_SEED, sizeof(bi->bytes) + RNG_SEED_LEN);

Likewise, this will destroy existing BI_AMIGA_CHIPSET and BI_MAC_SCCBASE
tags, breaking Amiga and Mac.

> +       if (getrandom(bi->bytes.data, RNG_SEED_LEN, GRND_NONBLOCK) != RNG_SEED_LEN) {
> +               bi_remove(BI_VIRT_RNG_SEED);
> +               return;
> +       }
> +       bi->bytes.len = cpu_to_be16(RNG_SEED_LEN);
> +}
> +
>

As random seeds can now be generic, I think it makes sense to introduce
a machine-independent BI_RND_SEED.
In hindsight, we should have done this from the beginning...

> --- a/kexec/arch/m68k/bootinfo.h
> +++ b/kexec/arch/m68k/bootinfo.h
> @@ -1,43 +1,49 @@
>  #include <asm/bootinfo.h>
> +#include <asm/bootinfo-virt.h>
>
>  #define DEFAULT_BOOTINFO_FILE  "/proc/bootinfo"
>  #define MAX_BOOTINFO_SIZE      1536
>
>
>      /*
>       *  Convenience overlay of several struct bi_record variants
>       */
>
>  struct bi_rec {
>         __be16 tag;
>         __be16 size;
>         union {
>                 __be32 data[0];
>                 /* shorthands for the types we use */
>                 __be32 machtype;
>                 struct {
>                         __be32 addr;
>                         __be32 size;
>                 } mem_info;
>                 char string[0];
> +               struct {
> +                       __be16 len;
> +                       u8 data[0];
> +               } bytes;

I'd rather call this rng_seed, to avoid confusion between "data"
and "bytes",

>         };
>  };

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds