Revert "random: block in /dev/urandom"

This reverts commit 6f98a4bfee.

It turns out we still can't do this.  Way too many platforms that don't
have any real source of randomness at boot and no jitter entropy because
they don't even have a cycle counter.

As reported by Guenter Roeck:

 "This causes a large number of qemu boot test failures for various
  architectures (arm, m68k, microblaze, sparc32, xtensa are the ones I
  observed).

  Common denominator is that boot hangs at 'Saving random seed:'"

This isn't hugely unexpected - we tried it, it failed, so now we'll
revert it.

Link: https://lore.kernel.org/all/20220322155820.GA1745955@roeck-us.net/
Reported-and-bisected-by: Guenter Roeck <linux@roeck-us.net>
Cc: Jason Donenfeld <Jason@zx2c4.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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, &random_fops, 0 },
+	 [9] = { "urandom", 0666, &urandom_fops, 0 },
 #ifdef CONFIG_PRINTK
 	[11] = { "kmsg", 0644, &kmsg_fops, 0 },
 #endif

drivers/char/random.c

@@ -89,14 +89,17 @@ static RAW_NOTIFIER_HEAD(random_ready_chain);
 /* 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");
 
 /*
  * Returns whether or not the input pool has been seeded and thus guaranteed
- * to supply cryptographically secure random numbers. This applies to
- * get_random_bytes() and get_random_{u32,u64,int,long}().
+ * to supply cryptographically secure random numbers. This applies to: the
+ * /dev/urandom device, the get_random_bytes function, and the get_random_{u32,
+ * ,u64,int,long} family of functions.
  *
  * Returns: true if the input pool has been seeded.
  *          false if the input pool has not been seeded.
@@ -112,10 +115,10 @@ static void try_to_generate_entropy(void);
 
 /*
  * Wait for the input pool to be seeded and thus guaranteed to supply
- * cryptographically secure random numbers. This applies to
- * get_random_bytes() and get_random_{u32,u64,int,long}(). Using any
- * of these functions without first calling this function means that
- * the returned numbers might not be cryptographically secure.
+ * cryptographically secure random numbers. This applies to: the /dev/urandom
+ * device, the get_random_bytes function, and the get_random_{u32,u64,int,long}
+ * family of functions. Using any of these functions without first calling
+ * this function forfeits the guarantee of security.
  *
  * Returns: 0 if the input pool has been seeded.
  *          -ERESTARTSYS if the function was interrupted by a signal.
@@ -220,10 +223,10 @@ static void _warn_unseeded_randomness(const char *func_name, void *caller, void
  *	unsigned long get_random_long()
  *
  * These interfaces will return the requested number of random bytes
- * into the given buffer or as a return value. The returned numbers are
- * the same as those of getrandom(0). The integer family of functions may
- * be higher performance for one-off random integers, because they do a
- * bit of buffering and do not invoke reseeding.
+ * into the given buffer or as a return value. This is equivalent to
+ * a read from /dev/urandom. The integer family of functions may be
+ * higher performance for one-off random integers, because they do a
+ * bit of buffering.
  *
  *********************************************************************/
 
@@ -300,6 +303,11 @@ static void crng_reseed(bool force)
 				  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;
+		}
 	}
 }
 
@@ -979,8 +987,10 @@ int __init rand_initialize(void)
 		pr_notice("crng init done (trusting CPU's manufacturer)\n");
 	}
 
-	if (ratelimit_disable)
+	if (ratelimit_disable) {
+		urandom_warning.interval = 0;
 		unseeded_warning.interval = 0;
+	}
 	return 0;
 }
 
@@ -1420,16 +1430,20 @@ 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 and /dev/urandom both have the same effect
- * as calling getrandom(2) with flags=0. (In earlier versions, however,
- * they each had different semantics.)
+ * 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.
  *
  * Writing to either /dev/random or /dev/urandom adds entropy to
  * the input pool but does not credit it.
  *
- * 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.
+ * Polling on /dev/random 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
@@ -1514,6 +1528,21 @@ 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)
 {
@@ -1600,6 +1629,15 @@ 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,
+};
+
 
 /********************************************************************
  *

include/linux/random.h

@@ -48,7 +48,7 @@ extern int unregister_random_ready_notifier(struct notifier_block *nb);
 extern size_t __must_check get_random_bytes_arch(void *buf, size_t nbytes);
 
 #ifndef MODULE
-extern const struct file_operations random_fops;
+extern const struct file_operations random_fops, urandom_fops;
 #endif
 
 u32 get_random_u32(void);