random: make credit_entropy_bits() always safe

commit a49c010 upstream.

This is called from various hwgenerator drivers, so rather than having
one "safe" version for userspace and one "unsafe" version for the
kernel, just make everything safe; the checks are cheap and sensible to
have anyway.

Reported-by: Sultan Alsawaf <sultan@kerneltoast.com>
Reviewed-by: Eric Biggers <ebiggers@google.com>
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>

drivers/char/random.c

@@ -447,18 +447,15 @@ static void process_random_ready_list(void)
 	spin_unlock_irqrestore(&random_ready_list_lock, flags);
 }
 
-/*
- * Credit (or debit) the entropy store with n bits of entropy.
- * Use credit_entropy_bits_safe() if the value comes from userspace
- * or otherwise should be checked for extreme values.
- */
 static void credit_entropy_bits(int nbits)
 {
 	int entropy_count, orig;
 
-	if (!nbits)
+	if (nbits <= 0)
 		return;
 
+	nbits = min(nbits, POOL_BITS);
+
 	do {
 		orig = READ_ONCE(input_pool.entropy_count);
 		entropy_count = min(POOL_BITS, orig + nbits);
@@ -470,18 +467,6 @@ static void credit_entropy_bits(int nbits)
 		crng_reseed(&primary_crng, true);
 }
 
-static int credit_entropy_bits_safe(int nbits)
-{
-	if (nbits < 0)
-		return -EINVAL;
-
-	/* Cap the value to avoid overflows */
-	nbits = min(nbits, POOL_BITS);
-
-	credit_entropy_bits(nbits);
-	return 0;
-}
-
 /*********************************************************************
  *
  * CRNG using CHACHA20
@@ -1526,7 +1511,10 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
 			return -EPERM;
 		if (get_user(ent_count, p))
 			return -EFAULT;
-		return credit_entropy_bits_safe(ent_count);
+		if (ent_count < 0)
+			return -EINVAL;
+		credit_entropy_bits(ent_count);
+		return 0;
 	case RNDADDENTROPY:
 		if (!capable(CAP_SYS_ADMIN))
 			return -EPERM;
@@ -1539,7 +1527,8 @@ static long random_ioctl(struct file *f, unsigned int cmd, unsigned long arg)
 		retval = write_pool((const char __user *)p, size);
 		if (retval < 0)
 			return retval;
-		return credit_entropy_bits_safe(ent_count);
+		credit_entropy_bits(ent_count);
+		return 0;
 	case RNDZAPENTCNT:
 	case RNDCLEARPOOL:
 		/*