Re: [PATCH v9 2/3] fdt: add support for rng-seed

[Kees Cook]

On Thu, Aug 29, 2019 at 06:03:57PM +0800, Hsin-Yi Wang wrote:
> On Thu, Aug 29, 2019 at 1:36 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >
> > Can this please be a boot param (with the default controlled by the
> > CONFIG)? See how CONFIG_RANDOM_TRUST_CPU is wired up...
> >
> > -Kees
> >
> 
> Currently rng-seed read and added in setup_arch() -->
> setup_machine_fdt().. -> early_init_dt_scan_chosen(), which is earlier
> than parse_early_param() that initializes early_param.
> 
> If we want to set it as a boot param, add_bootloader_randomness() can
> only be called after parse_early_param(). The seed can't be directly
> added to pool after it's read in. We need to store into global
> variable and load it later.
> If this seems okay then I'll add a patch for this. Thanks

This seems like a good idea to me.

> 
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -1096,13 +1096,15 @@ static const char *config_cmdline = CONFIG_CMDLINE;
> 
> +const void* rng_seed;
> +int rng_seed_len;

These should be __initdata, yes?

> +
>  int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
>                                                             int depth,
> void *data)
>  {
>         int l = 0;
>         const char *p = NULL;
>         char *cmdline = data;
> -       const void *rng_seed;
> 
>   pr_debug("search \"chosen\", depth: %d, uname: %s\n", depth, uname);
> 
> @@ -1137,10 +1139,8 @@ int __init early_init_dt_scan_chosen(unsigned
> long node, const char *uname,
> 
>          pr_debug("Command line is: %s\n", (char*)data);
> 
> -        rng_seed = of_get_flat_dt_prop(node, "rng-seed", &l);
> -        if (rng_seed && l > 0) {
> -                add_bootloader_randomness(rng_seed, l);  //
> Originally it's added to entropy pool here
> -
> +       rng_seed = of_get_flat_dt_prop(node, "rng-seed", &rng_seed_len);
> +       if (rng_seed && rng_seed_len > 0) {
>                 /* try to clear seed so it won't be found. */
> 
> diff --git a/include/linux/random.h b/include/linux/random.h
> index 831a002a1882..946840bba7c1 100644
> --- a/include/linux/random.h
> +++ b/include/linux/random.h
> @@ -31,6 +31,15 @@ static inline void add_latent_entropy(void)
>  static inline void add_latent_entropy(void) {}
>  #endif
> 
> +extern const void* rng_seed;
> +extern int rng_seed_len;
> +
> +static inline void add_bootloader_entropy(void)
> +{
> +        if (rng_seed && rng_seed_len > 0)
> +                add_bootloader_randomness(rng_seed, rng_seed_len);
> +}

And this should be __init

> +
>  extern void add_input_randomness(unsigned int type, unsigned int code,
>   unsigned int value) __latent_entropy;
>  extern void add_interrupt_randomness(int irq, int irq_flags) __latent_entropy;
> diff --git a/init/main.c b/init/main.c
> index 71847af32e4e..f74a8c7b34af 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -645,6 +645,7 @@ asmlinkage __visible void __init start_kernel(void)
>   * - adding command line entropy
>   */
>   rand_initialize();
> + add_bootloader_entropy();
>   add_latent_entropy();
>   add_device_randomness(command_line, strlen(command_line));
>   boot_init_stack_canary();

But yeah, looks reasonable to me.

-- 
Kees Cook