random: check for signals every PAGE_SIZE chunk of /dev/[u]random

commit e3c1c4fd9e6d14059ed93ebfe15e1c57793b1a05 upstream.

In 1448769c9cdb ("random: check for signal_pending() outside of
need_resched() check"), Jann pointed out that we previously were only
checking the TIF_NOTIFY_SIGNAL and TIF_SIGPENDING flags if the process
had TIF_NEED_RESCHED set, which meant in practice, super long reads to
/dev/[u]random would delay signal handling by a long time. I tried this
using the below program, and indeed I wasn't able to interrupt a
/dev/urandom read until after several megabytes had been read. The bug
he fixed has always been there, and so code that reads from /dev/urandom
without checking the return value of read() has mostly worked for a long
time, for most sizes, not just for <= 256.

Maybe it makes sense to keep that code working. The reason it was so
small prior, ignoring the fact that it didn't work anyway, was likely
because /dev/random used to block, and that could happen for pretty
large lengths of time while entropy was gathered. But now, it's just a
chacha20 call, which is extremely fast and is just operating on pure
data, without having to wait for some external event. In that sense,
/dev/[u]random is a lot more like /dev/zero.

Taking a page out of /dev/zero's read_zero() function, it always returns
at least one chunk, and then checks for signals after each chunk. Chunk
sizes there are of length PAGE_SIZE. Let's just copy the same thing for
/dev/[u]random, and check for signals and cond_resched() for every
PAGE_SIZE amount of data. This makes the behavior more consistent with
expectations, and should mitigate the impact of Jann's fix for the
age-old signal check bug.

---- test program ----

  #include <unistd.h>
  #include <signal.h>
  #include <stdio.h>
  #include <sys/random.h>

  static unsigned char x[~0U];

  static void handle(int) { }

  int main(int argc, char *argv[])
  {
    pid_t pid = getpid(), child;
    signal(SIGUSR1, handle);
    if (!(child = fork())) {
      for (;;)
        kill(pid, SIGUSR1);
    }
    pause();
    printf("interrupted after reading %zd bytes\n", getrandom(x, sizeof(x), 0));
    kill(child, SIGTERM);
    return 0;
  }

Cc: Jann Horn <jannh@google.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/random.c

@@ -523,9 +523,7 @@ EXPORT_SYMBOL(get_random_bytes);
 
 static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 {
-	bool large_request = nbytes > 256;
-	ssize_t ret = 0;
-	size_t len;
+	size_t len, left, ret = 0;
 	u32 chacha_state[CHACHA20_BLOCK_SIZE / sizeof(u32)];
 	u8 output[CHACHA20_BLOCK_SIZE];
 
@@ -537,46 +535,47 @@ static ssize_t get_random_bytes_user(void __user *buf, size_t nbytes)
 	 * bytes, in case userspace causes copy_to_user() below to sleep
 	 * forever, so that we still retain forward secrecy in that case.
 	 */
-	crng_make_state(chacha_state, (u8 *)&chacha_state[4], CHACHA_KEY_SIZE);
+	crng_make_state(chacha_state, (u8 *)&chacha_state[4], CHACHA20_KEY_SIZE);
 	/*
 	 * However, if we're doing a read of len <= 32, we don't need to
 	 * use chacha_state after, so we can simply return those bytes to
 	 * the user directly.
 	 */
-	if (nbytes <= CHACHA_KEY_SIZE) {
-		ret = copy_to_user(buf, &chacha_state[4], nbytes) ? -EFAULT : nbytes;
+	if (nbytes <= CHACHA20_KEY_SIZE) {
+		ret = nbytes - copy_to_user(buf, &chacha_state[4], nbytes);
 		goto out_zero_chacha;
 	}
 
-	do {
-		if (large_request) {
-			if (signal_pending(current)) {
-				if (!ret)
-					ret = -ERESTARTSYS;
-				break;
-			}
-			cond_resched();
-		}
-
+	for (;;) {
 		chacha20_block(chacha_state, output);
 		if (unlikely(chacha_state[12] == 0))
 			++chacha_state[13];
 
 		len = min_t(size_t, nbytes, CHACHA20_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;
-	} while (nbytes);
+		nbytes -= len;
+		if (!nbytes)
+			break;
+
+		BUILD_BUG_ON(PAGE_SIZE % CHACHA20_BLOCK_SIZE != 0);
+		if (ret % PAGE_SIZE == 0) {
+			if (signal_pending(current))
+				break;
+			cond_resched();
+		}
+	}
 
 	memzero_explicit(output, sizeof(output));
 out_zero_chacha:
 	memzero_explicit(chacha_state, sizeof(chacha_state));
-	return ret;
+	return ret ? ret : -EFAULT;
 }
 
 /*