random: fix crng_ready() test

The crng_init variable has three states:

0: The CRNG is not initialized at all
1: The CRNG has a small amount of entropy, hopefully good enough for
   early-boot, non-cryptographical use cases
2: The CRNG is fully initialized and we are sure it is safe for
   cryptographic use cases.

The crng_ready() function should only return true once we are in the
last state.  This addresses CVE-2018-1108.

Reported-by: Jann Horn <jannh@google.com>
Fixes: e192be9d9a30 ("random: replace non-blocking pool...")
Cc: stable@kernel.org # 4.8+
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jann Horn <jannh@google.com>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index e027e7fa1472b6612a50b597a9c3b13bf94939a8..c8ec1e70abde74947f5fd0fc108c15ceedb46179 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -427,7 +427,7 @@ struct crng_state primary_crng = {
  * its value (from 0->1->2).
  */
 static int crng_init = 0;
-#define crng_ready() (likely(crng_init > 0))
+#define crng_ready() (likely(crng_init > 1))
 static int crng_init_cnt = 0;
 #define CRNG_INIT_CNT_THRESH (2*CHACHA20_KEY_SIZE)
 static void _extract_crng(struct crng_state *crng,
@@ -794,7 +794,7 @@ static int crng_fast_load(const char *cp, size_t len)
 
        if (!spin_trylock_irqsave(&primary_crng.lock, flags))
                return 0;
-       if (crng_ready()) {
+       if (crng_init != 0) {
                spin_unlock_irqrestore(&primary_crng.lock, flags);
                return 0;
        }
@@ -856,7 +856,7 @@ static void _extract_crng(struct crng_state *crng,
 {
        unsigned long v, flags;
 
-       if (crng_init > 1 &&
+       if (crng_ready() &&
            time_after(jiffies, crng->init_time + CRNG_RESEED_INTERVAL))
                crng_reseed(crng, crng == &primary_crng ? &input_pool : NULL);
        spin_lock_irqsave(&crng->lock, flags);
@@ -1139,7 +1139,7 @@ void add_interrupt_randomness(int irq, int irq_flags)
        fast_mix(fast_pool);
        add_interrupt_bench(cycles);
 
-       if (!crng_ready()) {
+       if (unlikely(crng_init == 0)) {
                if ((fast_pool->count >= 64) &&
                    crng_fast_load((char *) fast_pool->pool,
                                   sizeof(fast_pool->pool))) {
@@ -2212,7 +2212,7 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 {
        struct entropy_store *poolp = &input_pool;
 
-       if (!crng_ready()) {
+       if (unlikely(crng_init == 0)) {
                crng_fast_load(buffer, count);
                return;
        }