[PATCH RFC v0] random: block in /dev/urandom

[Jason A. Donenfeld]

Hi Lennart,

On Mon, Feb 14, 2022 at 9:53 AM Lennart Poettering <mzxreary at 0pointer.de> wrote:
> So, systemd uses (potentially half-initialized) /dev/urandom for
> seeding its hash tables. For that its kinda OK if the random values
> have low entropy initially, as we'll automatically reseed when too
> many hash collisions happen, and then use a newer (and thus hopefully
> better) seed, again acquired through /dev/urandom. i.e. if the seeds
> are initially not good enough to thwart hash collision attacks, once
> the hash table are actually attacked we'll replace the seeds with
> someting better. For that all we need is that the random pool
> eventually gets better, that's all.
>
> So for that usecase /dev/urandom behaving the way it so far does is
> kinda nice.

Oh that's an interesting point. But that sounds to me like the problem
with this patch is not that it makes /dev/urandom block (its primary
purpose) but that it also removes GRND_INSECURE (a distraction). So
perhaps an improved patch would be something like the below, which
changes /dev/urandom for new kernels but doesn't remove GRND_INSECURE.
Then your hash tables could continue to use GRND_INSECURE and all would
be well.  (And for kernels without getrandom(), they'd just fall back to
/dev/urandom like normal which would have old semantics, so works.)

Jason



---------8<-----------------8<-------------------------------

diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index cc296f0823bd..9f586025dbe6 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -707,7 +707,7 @@ static const struct memdev {
 	 [5] = { "zero", 0666, &zero_fops, FMODE_NOWAIT },
 	 [7] = { "full", 0666, &full_fops, 0 },
 	 [8] = { "random", 0666, &random_fops, 0 },
-	 [9] = { "urandom", 0666, &urandom_fops, 0 },
+	 [9] = { "urandom", 0666, &random_fops, 0 },
 #ifdef CONFIG_PRINTK
 	[11] = { "kmsg", 0644, &kmsg_fops, 0 },
 #endif
diff --git a/drivers/char/random.c b/drivers/char/random.c
index ce199af9bc56..ae4400c48b2f 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -89,8 +89,6 @@ static LIST_HEAD(random_ready_list);
 /* Control how we warn userspace. */
 static struct ratelimit_state unseeded_warning =
 	RATELIMIT_STATE_INIT("warn_unseeded_randomness", HZ, 3);
-static struct ratelimit_state urandom_warning =
-	RATELIMIT_STATE_INIT("warn_urandom_randomness", HZ, 3);
 static int ratelimit_disable __read_mostly;
 module_param_named(ratelimit_disable, ratelimit_disable, int, 0644);
 MODULE_PARM_DESC(ratelimit_disable, "Disable random ratelimit suppression");
@@ -336,11 +334,6 @@ static void crng_reseed(void)
 				  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;
-		}
 	}
 }

@@ -993,10 +986,8 @@ int __init rand_initialize(void)
 		pr_notice("crng init done (trusting CPU's manufacturer)\n");
 	}

-	if (ratelimit_disable) {
-		urandom_warning.interval = 0;
+	if (ratelimit_disable)
 		unseeded_warning.interval = 0;
-	}
 	return 0;
 }

@@ -1387,20 +1378,17 @@ static void try_to_generate_entropy(void)
  * getrandom(2) is the primary modern interface into the RNG and should
  * be used in preference to anything else.
  *
- * Reading from /dev/random has the same functionality as calling
- * getrandom(2) with flags=0. In earlier versions, however, it had
- * vastly different semantics and should therefore be avoided, to
- * prevent backwards compatibility issues.
- *
- * Reading from /dev/urandom has the same functionality as calling
- * getrandom(2) with flags=GRND_INSECURE. Because it does not block
- * waiting for the RNG to be ready, it should not be used.
+ * Reading from /dev/random and /dev/urandom both the same effect as
+ * calling getrandom(2) with flags=0. In earlier versions, however,
+ * they each had vastly different semantics and should therefore be
+ * avoided to prevent backwards compatibility issues.
  *
  * Writing to either /dev/random or /dev/urandom adds entropy to
  * the input pool but does not credit it.
  *
- * Polling on /dev/random indicates when the RNG is initialized, on
- * the read side, and when it wants new entropy, on the write side.
+ * Polling on /dev/random or /dev/urandom indicates when the RNG
+ * is initialized, on the read side, and when it wants new entropy,
+ * on the write side.
  *
  * Both /dev/random and /dev/urandom have the same set of ioctls for
  * adding entropy, getting the entropy count, zeroing the count, and
@@ -1485,21 +1473,6 @@ static ssize_t random_write(struct file *file, const char __user *buffer,
 	return (ssize_t)count;
 }

-static ssize_t urandom_read(struct file *file, char __user *buf, size_t nbytes,
-			    loff_t *ppos)
-{
-	static int maxwarn = 10;
-
-	if (!crng_ready() && maxwarn > 0) {
-		maxwarn--;
-		if (__ratelimit(&urandom_warning))
-			pr_notice("%s: uninitialized urandom read (%zd bytes read)\n",
-				  current->comm, nbytes);
-	}
-
-	return get_random_bytes_user(buf, nbytes);
-}
-
 static ssize_t random_read(struct file *file, char __user *buf, size_t nbytes,
 			   loff_t *ppos)
 {
@@ -1586,15 +1559,6 @@ const struct file_operations random_fops = {
 	.llseek = noop_llseek,
 };

-const struct file_operations urandom_fops = {
-	.read = urandom_read,
-	.write = random_write,
-	.unlocked_ioctl = random_ioctl,
-	.compat_ioctl = compat_ptr_ioctl,
-	.fasync = random_fasync,
-	.llseek = noop_llseek,
-};
-

 /********************************************************************
  *