Re: [PATCH 2/4] x86/boot: Allow a "silent" kaslr random byte fetch

[Nick Desaulniers <ndesaulniers@google.com>]

On Wed, Oct 13, 2021 at 10:57 AM Kees Cook <keescook@chromium.org> wrote:
>
> Under earlyprintk, each RNG call produces a debug report line. To support
> the future FGKASLR feature, which will fetch random bytes during function
> shuffling, this is not useful information (each line is identical and
> tells us nothing new), needlessly spamming the console. Instead, allow
> for a NULL "purpose" to suppress the debug reporting.

Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  arch/x86/lib/kaslr.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/lib/kaslr.c b/arch/x86/lib/kaslr.c
> index a53665116458..2b3eb8c948a3 100644
> --- a/arch/x86/lib/kaslr.c
> +++ b/arch/x86/lib/kaslr.c
> @@ -56,11 +56,14 @@ unsigned long kaslr_get_random_long(const char *purpose)
>         unsigned long raw, random = get_boot_seed();
>         bool use_i8254 = true;
>
> -       debug_putstr(purpose);
> -       debug_putstr(" KASLR using");
> +       if (purpose) {
> +               debug_putstr(purpose);
> +               debug_putstr(" KASLR using");
> +       }
>
>         if (has_cpuflag(X86_FEATURE_RDRAND)) {
> -               debug_putstr(" RDRAND");
> +               if (purpose)
> +                       debug_putstr(" RDRAND");
>                 if (rdrand_long(&raw)) {
>                         random ^= raw;
>                         use_i8254 = false;
> @@ -68,7 +71,8 @@ unsigned long kaslr_get_random_long(const char *purpose)
>         }
>
>         if (has_cpuflag(X86_FEATURE_TSC)) {
> -               debug_putstr(" RDTSC");
> +               if (purpose)
> +                       debug_putstr(" RDTSC");
>                 raw = rdtsc();
>
>                 random ^= raw;
> @@ -76,7 +80,8 @@ unsigned long kaslr_get_random_long(const char *purpose)
>         }
>
>         if (use_i8254) {
> -               debug_putstr(" i8254");
> +               if (purpose)
> +                       debug_putstr(" i8254");
>                 random ^= i8254();
>         }
>
> @@ -86,7 +91,8 @@ unsigned long kaslr_get_random_long(const char *purpose)
>             : "a" (random), "rm" (mix_const));
>         random += raw;
>
> -       debug_putstr("...\n");
> +       if (purpose)
> +               debug_putstr("...\n");
>
>         return random;
>  }
> --
> 2.30.2
>


-- 
Thanks,
~Nick Desaulniers