linux-kernel - Re: [PATCH] mips: allow firmware to pass RNG seed to kernel

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Date:   Tue, 4 Oct 2022 00:07:08 +0200
From:   Philippe Mathieu-Daudé <f4bug@...at.org>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mips: allow firmware to pass RNG seed to kernel

Hi Jason,

On Fri, Sep 30, 2022 at 4:05 PM Jason A. Donenfeld <Jason@...c4.com> wrote:
>
> Nearly all other firmware environments have some way of passing a RNG
> seed to initialize the RNG: DTB's rng-seed, EFI's RNG protocol, m68k's
> bootinfo block, x86's setup_data, and so forth. This adds something
> similar for MIPS, which will allow various firmware environments,
> bootloaders, and hypervisors to pass an RNG seed to initialize the
> kernel's RNG.
>
> Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> ---
>  arch/mips/kernel/setup.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/arch/mips/kernel/setup.c b/arch/mips/kernel/setup.c
> index 2ca156a5b231..39c79f67c7a3 100644
> --- a/arch/mips/kernel/setup.c
> +++ b/arch/mips/kernel/setup.c
> @@ -42,6 +42,7 @@
>  #include <asm/setup.h>
>  #include <asm/smp-ops.h>
>  #include <asm/prom.h>
> +#include <asm/fw/fw.h>
>
>  #ifdef CONFIG_MIPS_ELF_APPENDED_DTB
>  char __section(".appended_dtb") __appended_dtb[0x100000];
> @@ -756,6 +757,24 @@ static void __init prefill_possible_map(void)
>  static inline void prefill_possible_map(void) {}
>  #endif
>
> +static void __init setup_rng_seed(void)
> +{
> +       char *rng_seed_hex = fw_getenv("rngseed");
> +       u8 rng_seed[512];
> +       size_t len;
> +
> +       if (!rng_seed_hex)
> +               return;
> +

Assuming rngseed="x", ...

> +       len = min(sizeof(rng_seed), strlen(rng_seed_hex) / 2);

... len = 0 ...

> +       if (hex2bin(rng_seed, rng_seed_hex, len))
> +               return;

hex2bin(..., len=0) = 0

> +
> +       add_bootloader_randomness(rng_seed, len);

So we call char/random code with len=0. Is it safe?
Maybe simply safer to check len before calling hex2bin?

> +       memzero_explicit(rng_seed, len);
> +       memzero_explicit(rng_seed_hex, len * 2);
> +}
> +
>  void __init setup_arch(char **cmdline_p)
>  {
>         cpu_probe();
> @@ -786,6 +805,8 @@ void __init setup_arch(char **cmdline_p)
>         paging_init();
>
>         memblock_dump_all();
> +
> +       setup_rng_seed();
>  }
>
>  unsigned long kernelsp[NR_CPUS];
> --
> 2.37.3
>