random: use the architectural HWRNG for the SHA's IV in extract_buf()

To help assuage the fears of those who think the NSA can introduce a
massive hack into the instruction decode and out of order execution
engine in the CPU without hundreds of Intel engineers knowing about
it (only one of which woud need to have the conscience and courage of
Edward Snowden to spill the beans to the public), use the HWRNG to
initialize the SHA starting value, instead of xor'ing it in
afterwards.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

drivers/char/random.c

@@ -1012,23 +1012,23 @@ static void extract_buf(struct entropy_store *r, __u8 *out)
 	__u8 extract[64];
 	unsigned long flags;
 
-	/* Generate a hash across the pool, 16 words (512 bits) at a time */
-	sha_init(hash.w);
-	spin_lock_irqsave(&r->lock, flags);
-	for (i = 0; i < r->poolinfo->poolwords; i += 16)
-		sha_transform(hash.w, (__u8 *)(r->pool + i), workspace);
-
 	/*
 	 * If we have an architectural hardware random number
-	 * generator, mix that in, too.
+	 * generator, use it for SHA's initial vector
 	 */
+	sha_init(hash.w);
 	for (i = 0; i < LONGS(20); i++) {
 		unsigned long v;
 		if (!arch_get_random_long(&v))
 			break;
-		hash.l[i] ^= v;
+		hash.l[i] = v;
 	}
 
+	/* Generate a hash across the pool, 16 words (512 bits) at a time */
+	spin_lock_irqsave(&r->lock, flags);
+	for (i = 0; i < r->poolinfo->poolwords; i += 16)
+		sha_transform(hash.w, (__u8 *)(r->pool + i), workspace);
+
 	/*
 	 * We mix the hash back into the pool to prevent backtracking
 	 * attacks (where the attacker knows the state of the pool