[PATCH 3/9] x86/boot: Allow a "silent" kaslr random byte fetch

Kristen Carlson Accardi
2020-04-15T21:05:53Z

From: Kees Cook <keescook@chromium.org>

Under earlyprintk, each RNG call produces a debug report line. When
shuffling hundreds of functions, this is not useful information (each
line is identical and tells us nothing new). Instead, allow for a NULL
"purpose" to suppress the debug reporting.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
---
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;
}
--
2.20.1