drivers/char/random.c: fix a race which can lead to a bogus BUG()

Fix a bug reported by and diagnosed by Aaron Straus.

This is a regression intruduced into 2.6.26 by

    commit adc782d
    Author: Matt Mackall <mpm@selenic.com>
    Date:   Tue Apr 29 01:03:07 2008 -0700

        random: simplify and rename credit_entropy_store

credit_entropy_bits() does:

	spin_lock_irqsave(&r->lock, flags);
	...
	if (r->entropy_count > r->poolinfo->POOLBITS)
		r->entropy_count = r->poolinfo->POOLBITS;

so there is a time window in which this BUG_ON():

static size_t account(struct entropy_store *r, size_t nbytes, int min,
		      int reserved)
{
	unsigned long flags;

	BUG_ON(r->entropy_count > r->poolinfo->POOLBITS);

	/* Hold lock while accounting */
	spin_lock_irqsave(&r->lock, flags);

can trigger.

We could fix this by moving the assertion inside the lock, but it seems
safer and saner to revert to the old behaviour wherein
entropy_store.entropy_count at no time exceeds
entropy_store.poolinfo->POOLBITS.

Reported-by: Aaron Straus <aaron@merfinllc.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: <stable@kernel.org>		[2.6.26.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/random.c

@@ -407,7 +407,7 @@ struct entropy_store {
 	/* read-write data: */
 	spinlock_t lock;
 	unsigned add_ptr;
-	int entropy_count;
+	int entropy_count;	/* Must at no time exceed ->POOLBITS! */
 	int input_rotate;
 };
 
@@ -520,27 +520,28 @@ static void mix_pool_bytes(struct entropy_store *r, const void *in, int bytes)
 static void credit_entropy_bits(struct entropy_store *r, int nbits)
 {
 	unsigned long flags;
+	int entropy_count;
 
 	if (!nbits)
 		return;
 
 	spin_lock_irqsave(&r->lock, flags);
 
 	DEBUG_ENT("added %d entropy credits to %s\n", nbits, r->name);
-	r->entropy_count += nbits;
-	if (r->entropy_count < 0) {
+	entropy_count = r->entropy_count;
+	entropy_count += nbits;
+	if (entropy_count < 0) {
 		DEBUG_ENT("negative entropy/overflow\n");
-		r->entropy_count = 0;
-	} else if (r->entropy_count > r->poolinfo->POOLBITS)
-		r->entropy_count = r->poolinfo->POOLBITS;
+		entropy_count = 0;
+	} else if (entropy_count > r->poolinfo->POOLBITS)
+		entropy_count = r->poolinfo->POOLBITS;
+	r->entropy_count = entropy_count;
 
 	/* should we wake readers? */
-	if (r == &input_pool &&
-	    r->entropy_count >= random_read_wakeup_thresh) {
+	if (r == &input_pool && entropy_count >= random_read_wakeup_thresh) {
 		wake_up_interruptible(&random_read_wait);
 		kill_fasync(&fasync, SIGIO, POLL_IN);
 	}
-
 	spin_unlock_irqrestore(&r->lock, flags);
 }