From 43838a23a05fbd13e47d750d3dfd77001536dd33 Mon Sep 17 00:00:00 2001
From: Theodore Ts'o <tytso@mit.edu>
Date: Wed, 11 Apr 2018 13:27:52 -0400
Subject: [PATCH] random: fix crng_ready() test
Git-commit: 43838a23a05fbd13e47d750d3dfd77001536dd33
Patch-mainline: v4.17-rc2
References: CVE-2018-1108,bsc#1090818

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>
Acked-by: Takashi Iwai <tiwai@suse.de>

---
 drivers/char/random.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -429,7 +429,7 @@ 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
 
 	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_st
 {
 	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);
@@ -1142,7 +1142,7 @@ void add_interrupt_randomness(int irq, i
 	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))) {
@@ -2180,7 +2180,7 @@ void add_hwgenerator_randomness(const ch
 {
 	struct entropy_store *poolp = &input_pool;
 
-	if (!crng_ready()) {
+	if (unlikely(crng_init == 0)) {
 		crng_fast_load(buffer, count);
 		return;
 	}