random: allow partial reads if later user copies fail

Rather than failing entirely if a copy_to_user() fails at some point,
instead we should return a partial read for the amount that succeeded
prior, unless none succeeded at all, in which case we return -EFAULT as
before.

This makes it consistent with other reader interfaces. For example, the
following snippet for /dev/zero outputs "4" followed by "1":

  int fd;
  void *x = mmap(NULL, 4096, PROT_WRITE, MAP_ANONYMOUS | MAP_PRIVATE, -1, 0);
  assert(x != MAP_FAILED);
  fd = open("/dev/zero", O_RDONLY);
  assert(fd >= 0);
  printf("%zd\n", read(fd, x, 4));
  printf("%zd\n", read(fd, x + 4095, 4));
  close(fd);

This brings that same standard behavior to the various RNG reader
interfaces.

While we're at it, we can streamline the loop logic a little bit.
Suggested-by: NLinus Torvalds <torvalds@linux-foundation.org>
Cc: Jann Horn <jannh@google.com>
Signed-off-by: NJason A. Donenfeld <Jason@zx2c4.com>

drivers/char/random.c

@@ -523,8 +523,7 @@ EXPORT_SYMBOL(get_random_bytes);
 
 static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 {
-	ssize_t ret = 0;
-	size_t len;
+	size_t len, left, ret = 0;
 	u32 chacha_state[CHACHA_STATE_WORDS];
 	u8 output[CHACHA_BLOCK_SIZE];
 
@@ -543,37 +542,40 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 	 * the user directly.
 	 */
 	if (nbytes <= CHACHA_KEY_SIZE) {
-		ret = copy_to_user(buf, &chacha_state[4], nbytes) ? -EFAULT : nbytes;
+		ret = nbytes - copy_to_user(buf, &chacha_state[4], nbytes);
 		goto out_zero_chacha;
 	}
 
-	do {
+	for (;;) {
 		chacha20_block(chacha_state, output);
 		if (unlikely(chacha_state[12] == 0))
 			++chacha_state[13];
 
 		len = min_t(size_t, nbytes, CHACHA_BLOCK_SIZE);
-		if (copy_to_user(buf, output, len)) {
-			ret = -EFAULT;
+		left = copy_to_user(buf, output, len);
+		if (left) {
+			ret += len - left;
 			break;
 		}
 
-		nbytes -= len;
 		buf += len;
 		ret += len;
+		nbytes -= len;
+		if (!nbytes)
+			break;
 
 		BUILD_BUG_ON(PAGE_SIZE % CHACHA_BLOCK_SIZE != 0);
-		if (!(ret % PAGE_SIZE) && nbytes) {
+		if (ret % PAGE_SIZE == 0) {
 			if (signal_pending(current))
 				break;
 			cond_resched();
 		}
-	} while (nbytes);
+	}
 
 	memzero_explicit(output, sizeof(output));
 out_zero_chacha:
 	memzero_explicit(chacha_state, sizeof(chacha_state));
-	return ret;
+	return ret ? ret : -EFAULT;
 }
 
 /*