Re: [PATCH] arm64: add support for rng-seed

[Hsin-Yi Wang]

On Tue, May 7, 2019 at 12:54 PM Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
>
> Introducing a chosen node, rng-seed, which is an 64 bytes entropy
> that can be passed to kernel called very early to increase device
> randomness. Bootloader should provide this entropy and the value is
> read from /chosen/rng-seed in DT.
>
> Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
>
> ---
>  Documentation/devicetree/bindings/chosen.txt | 14 +++++++++
>  arch/arm64/kernel/setup.c                    |  2 ++
>  drivers/of/fdt.c                             | 33 ++++++++++++++++++++
>  include/linux/of_fdt.h                       |  1 +
>  4 files changed, 50 insertions(+)
>
> diff --git a/Documentation/devicetree/bindings/chosen.txt b/Documentation/devicetree/bindings/chosen.txt
> index 45e79172a646..bfd360691650 100644
> --- a/Documentation/devicetree/bindings/chosen.txt
> +++ b/Documentation/devicetree/bindings/chosen.txt
> @@ -28,6 +28,20 @@ mode) when EFI_RNG_PROTOCOL is supported, it will be overwritten by
>  the Linux EFI stub (which will populate the property itself, using
>  EFI_RNG_PROTOCOL).
>
> +rng-seed
> +-----------
> +
> +This property served as an entropy to add device randomness. It is parsed
> +as a 64 byte value, e.g.
> +
> +/ {
> +       chosen {
> +               rng-seed = <0x31951b3c 0xc9fab3a5 0xffdf1660 ...>
> +       };
> +};
> +
> +This random value should be provided by bootloader.
> +
>  stdout-path
>  -----------
>
> diff --git a/arch/arm64/kernel/setup.c b/arch/arm64/kernel/setup.c
> index 413d566405d1..ade4261516dd 100644
> --- a/arch/arm64/kernel/setup.c
> +++ b/arch/arm64/kernel/setup.c
> @@ -292,6 +292,8 @@ void __init setup_arch(char **cmdline_p)
>         early_fixmap_init();
>         early_ioremap_init();
>
> +       early_init_dt_rng_seed(__fdt_pointer);
Currently this can only be called before setup_machine_fdt(). Since
setup_machine_fdt() called fixmap_remap_fdt() //
__fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL_RO), we can't modify DT
after that. And rng-seed needs to be wiped out after read.
Another option is to called earlier, at arch/arm64/kernel/head.S,
similar to kaslr_early_init.

> +
>         setup_machine_fdt(__fdt_pointer);
>
>         parse_early_param();
> diff --git a/drivers/of/fdt.c b/drivers/of/fdt.c
> index de893c9616a1..74e2c0c80b91 100644
> --- a/drivers/of/fdt.c
> +++ b/drivers/of/fdt.c
> @@ -22,6 +22,7 @@
>  #include <linux/slab.h>
>  #include <linux/libfdt.h>
>  #include <linux/debugfs.h>
> +#include <linux/random.h>
>  #include <linux/serial_core.h>
>  #include <linux/sysfs.h>
>
> @@ -1117,6 +1118,38 @@ int __init early_init_dt_scan_chosen(unsigned long node, const char *uname,
>         return 1;
>  }
>
> +extern void *__init __fixmap_remap_fdt(phys_addr_t dt_phys, int *size,
> +                                      pgprot_t prot);
> +
> +void __init early_init_dt_rng_seed(u64 dt_phys)
> +{
> +       void *fdt;
> +       int node, size, i;
> +       fdt64_t *prop;
> +       u64 rng_seed[8];
> +
> +       fdt = __fixmap_remap_fdt(dt_phys, &size, PAGE_KERNEL);
> +       if (!fdt)
> +               return;
> +
> +       node = fdt_path_offset(fdt, "/chosen");
> +       if (node < 0)
> +               return;
> +
> +       prop = fdt_getprop_w(fdt, node, "rng-seed", &size);
> +       if (!prop || size != sizeof(u64) * 8)
> +               return;
> +
> +       for (i = 0; i < 8; i++) {
> +               rng_seed[i] = fdt64_to_cpu(*(prop + i));
> +               /* clear seed so it won't be found. */
> +               *(prop + i) = 0;
> +       }
> +       add_device_randomness(rng_seed, size);
> +
> +       return;
> +}
> +
>  #ifndef MIN_MEMBLOCK_ADDR
>  #define MIN_MEMBLOCK_ADDR      __pa(PAGE_OFFSET)
>  #endif
> diff --git a/include/linux/of_fdt.h b/include/linux/of_fdt.h
> index a713e5d156d8..a4548dd6351e 100644
> --- a/include/linux/of_fdt.h
> +++ b/include/linux/of_fdt.h
> @@ -71,6 +71,7 @@ extern uint32_t of_get_flat_dt_phandle(unsigned long node);
>
>  extern int early_init_dt_scan_chosen(unsigned long node, const char *uname,
>                                      int depth, void *data);
> +extern void early_init_dt_rng_seed(u64 dt_phys);
>  extern int early_init_dt_scan_memory(unsigned long node, const char *uname,
>                                      int depth, void *data);
>  extern int early_init_dt_scan_chosen_stdout(void);
> --
> 2.20.1
>