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

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