Re: [PATCH qemu] mips/malta: pass RNG seed to to kernel via env var

[Daniel P . Berrangé]

On Fri, Sep 30, 2022 at 04:05:20PM +0200, Jason A. Donenfeld wrote:
> With the kernel patch linked below, Linux ingests a RNG seed
> passed from the hypervisor. So, pass this for the Malta platform, and
> reinitialize it on reboot too, so that it's always fresh.
> 
> Link: 
> https://lore.kernel.org/linux-mips/20220930140138.575751-1-Jason@zx2c4.com/
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>  hw/mips/malta.c | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/hw/mips/malta.c b/hw/mips/malta.c
> index 0e932988e0..9d793b3c17 100644
> --- a/hw/mips/malta.c
> +++ b/hw/mips/malta.c
> @@ -26,6 +26,7 @@
>  #include "qemu/units.h"
>  #include "qemu/bitops.h"
>  #include "qemu/datadir.h"
> +#include "qemu/guest-random.h"
>  #include "hw/clock.h"
>  #include "hw/southbridge/piix.h"
>  #include "hw/isa/superio.h"
> @@ -1017,6 +1018,17 @@ static void G_GNUC_PRINTF(3, 4) prom_set(uint32_t 
> *prom_buf, int index,
>      va_end(ap);
>  }
>  
> +static void reinitialize_rng_seed(void *opaque)
> +{
> +    char *rng_seed_hex = opaque;
> +    uint8_t rng_seed[32];
> +
> +    qemu_guest_getrandom_nofail(rng_seed, sizeof(rng_seed));
> +    for (size_t i = 0; i < sizeof(rng_seed); ++i) {
> +        sprintf(rng_seed_hex + i * 2, "%02x", rng_seed[i]);
> +    }
> +}
> +
>  /* Kernel */
>  static uint64_t load_kernel(void)
>  {
> @@ -1028,6 +1040,8 @@ static uint64_t load_kernel(void)
>      long prom_size;
>      int prom_index = 0;
>      uint64_t (*xlate_to_kseg0) (void *opaque, uint64_t addr);
> +    uint8_t rng_seed[32];
> +    char rng_seed_hex[sizeof(rng_seed) * 2 + 1];

The +1 suggests you're intending to NUL terminate, but nothing
in this patch ever initializes this last byte.

>  
>  #if TARGET_BIG_ENDIAN
>      big_endian = 1;
> @@ -1115,9 +1129,20 @@ static uint64_t load_kernel(void)
>  
>      prom_set(prom_buf, prom_index++, "modetty0");
>      prom_set(prom_buf, prom_index++, "38400n8r");
> +
> +    qemu_guest_getrandom_nofail(rng_seed, sizeof(rng_seed));
> +    for (size_t i = 0; i < sizeof(rng_seed); ++i) {
> +        sprintf(rng_seed_hex + i * 2, "%02x", rng_seed[i]);
> +    }
> +    prom_set(prom_buf, prom_index++, "rngseed");
> +    prom_set(prom_buf, prom_index++, "%s", rng_seed_hex);
> +
>      prom_set(prom_buf, prom_index++, NULL);
>  
>      rom_add_blob_fixed("prom", prom_buf, prom_size, ENVP_PADDR);
> +    qemu_register_reset(reinitialize_rng_seed,
> +                        memmem(rom_ptr(ENVP_PADDR, prom_size), prom_size,
> +                               rng_seed_hex, sizeof(rng_seed_hex)));
>  
>      g_free(prom_buf);
>      return kernel_entry;
> -- 
> 2.37.3
> 
> 

With regards,
Daniel
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