[RFC PATCH 25/41] random: probe cycle counter resolution at initialization

[Nicolai Stange]

An upcoming patch will change the entropy estimate per
add_interrupt_randomness() event for fips_enabled based on whether
random_get_entropy() resp. get_cycles() is able to capture individual
instructions.

For example, x86's TSC would qualify, whereas I've seen cycle counters on
e.g. a Raspberry PI 2B with an advertised resolution of only 52ns even
though the CPU had been clocked at 1GHz. And then there's possibly hardware
which doesn't have a cycle counter at all and where get_cycles() would
always return the same constant.

Make rand_initialize() probe the cycle counter resolution.

Introduce a new static_key have_highres_cycle_ctr, indicicating whether
or not the system's cycle counter is able to capture individual
instructions. Initially it's set to true. Introduce
probe_cycle_ctr_resolution() and call it from rand_initialize().
Make probe_cycle_ctr_resolution() compare 16 successive
random_get_entropy() values and disable have_highres_cycle_ctr in case
the same value has been read two times in a row. As have_highres_cycle_ctr
will be only accessed if fips_enabled is true, make it return early in
case it's not set.

Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
---
 drivers/char/random.c | 29 +++++++++++++++++++++++++++++
 1 file changed, 29 insertions(+)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index aaddee4e4ab1..a985ceb22c7c 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -335,6 +335,7 @@
 #include <linux/syscalls.h>
 #include <linux/completion.h>
 #include <linux/uuid.h>
+#include <linux/jump_label.h>
 #include <crypto/chacha.h>
 #include <crypto/sha.h>
 
@@ -478,6 +479,8 @@ static struct ratelimit_state urandom_warning =
 
 static int ratelimit_disable __read_mostly;
 
+static DEFINE_STATIC_KEY_TRUE(have_highres_cycle_ctr);
+
 module_param_named(ratelimit_disable, ratelimit_disable, int, 0644);
 MODULE_PARM_DESC(ratelimit_disable, "Disable random ratelimit suppression");
 
@@ -2170,6 +2173,31 @@ static void __init init_std_data(struct entropy_store *r)
 	mix_pool_bytes(r, utsname(), sizeof(*(utsname())));
 }
 
+static void probe_cycle_ctr_resolution(void)
+{
+	cycles_t prev;
+	int i;
+
+	if (!fips_enabled)
+		return;
+
+	/*
+	 * Check if the cycle counter has insn granularity (or at
+	 * least close to).
+	 */
+	prev = random_get_entropy();
+	for (i = 0; i < 16; ++i) {
+		cycles_t next;
+
+		next = random_get_entropy();
+		if (next == prev) {
+			static_branch_disable(&have_highres_cycle_ctr);
+			return;
+		}
+		prev = next;
+	}
+}
+
 /*
  * Note that setup_arch() may call add_device_randomness()
  * long before we get here. This allows seeding of the pools
@@ -2182,6 +2210,7 @@ static void __init init_std_data(struct entropy_store *r)
  */
 int __init rand_initialize(void)
 {
+	probe_cycle_ctr_resolution();
 	init_std_data(&input_pool);
 	crng_initialize_primary(&primary_crng);
 	crng_global_init_time = jiffies;
-- 
2.26.2