random: avoid initializing twice in credit race

commit fed7ef061686cc813b1f3d8d0edc6c35b4d3537b upstream.

Since all changes of crng_init now go through credit_init_bits(), we can
fix a long standing race in which two concurrent callers of
credit_init_bits() have the new bit count >= some threshold, but are
doing so with crng_init as a lower threshold, checked outside of a lock,
resulting in crng_reseed() or similar being called twice.

In order to fix this, we can use the original cmpxchg value of the bit
count, and only change crng_init when the bit count transitions from
below a threshold to meeting the threshold.

Reviewed-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/char/random.c b/drivers/char/random.c
index b507c3b003a3c8eb8498f7cae7cffd7bc78820aa..0e1a99bc72fc01c5d81aebafb1733fb0cca907d7 100644 (file)
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -264,7 +264,6 @@ static void crng_reseed(void)
        unsigned long flags;
        unsigned long next_gen;
        u8 key[CHACHA20_KEY_SIZE];
-       bool finalize_init = false;
 
        extract_entropy(key, sizeof(key));
 
@@ -281,28 +280,10 @@ static void crng_reseed(void)
                ++next_gen;
        WRITE_ONCE(base_crng.generation, next_gen);
        WRITE_ONCE(base_crng.birth, jiffies);
-       if (!crng_ready()) {
+       if (!crng_ready())
                crng_init = CRNG_READY;
-               finalize_init = true;
-       }
        spin_unlock_irqrestore(&base_crng.lock, flags);
        memzero_explicit(key, sizeof(key));
-       if (finalize_init) {
-               process_random_ready_list();
-               wake_up_interruptible(&crng_init_wait);
-               kill_fasync(&fasync, SIGIO, POLL_IN);
-               pr_notice("crng init done\n");
-               if (unseeded_warning.missed) {
-                       pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
-                                 unseeded_warning.missed);
-                       unseeded_warning.missed = 0;
-               }
-               if (urandom_warning.missed) {
-                       pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
-                                 urandom_warning.missed);
-                       urandom_warning.missed = 0;
-               }
-       }
 }
 
 /*
@@ -818,7 +799,7 @@ static void extract_entropy(void *buf, size_t nbytes)
 
 static void credit_init_bits(size_t nbits)
 {
-       unsigned int init_bits, orig, add;
+       unsigned int new, orig, add;
        unsigned long flags;
 
        if (crng_ready() || !nbits)
@@ -828,13 +809,28 @@ static void credit_init_bits(size_t nbits)
 
        do {
                orig = READ_ONCE(input_pool.init_bits);
-               init_bits = min_t(unsigned int, POOL_BITS, orig + add);
-       } while (cmpxchg(&input_pool.init_bits, orig, init_bits) != orig);
+               new = min_t(unsigned int, POOL_BITS, orig + add);
+       } while (cmpxchg(&input_pool.init_bits, orig, new) != orig);
 
-       if (!crng_ready() && init_bits >= POOL_READY_BITS)
-               crng_reseed();
-       else if (unlikely(crng_init == CRNG_EMPTY && init_bits >= POOL_EARLY_BITS)) {
+       if (orig < POOL_READY_BITS && new >= POOL_READY_BITS) {
+               crng_reseed(); /* Sets crng_init to CRNG_READY under base_crng.lock. */
+               process_random_ready_list();
+               wake_up_interruptible(&crng_init_wait);
+               kill_fasync(&fasync, SIGIO, POLL_IN);
+               pr_notice("crng init done\n");
+               if (unseeded_warning.missed) {
+                       pr_notice("%d get_random_xx warning(s) missed due to ratelimiting\n",
+                                 unseeded_warning.missed);
+                       unseeded_warning.missed = 0;
+               }
+               if (urandom_warning.missed) {
+                       pr_notice("%d urandom warning(s) missed due to ratelimiting\n",
+                                 urandom_warning.missed);
+                       urandom_warning.missed = 0;
+               }
+       } else if (orig < POOL_EARLY_BITS && new >= POOL_EARLY_BITS) {
                spin_lock_irqsave(&base_crng.lock, flags);
+               /* Check if crng_init is CRNG_EMPTY, to avoid race with crng_reseed(). */
                if (crng_init == CRNG_EMPTY) {
                        extract_entropy(base_crng.key, sizeof(base_crng.key));
                        crng_init = CRNG_EARLY;