random: check for increase of entropy_count because of signed conversion

The expression entropy_count -= ibytes << (ENTROPY_SHIFT + 3) could
actually increase entropy_count if during assignment of the unsigned
expression on the RHS (mind the -=) we reduce the value modulo
2^width(int) and assign it to entropy_count. Trinity found this.

[ Commit modified by tytso to add an additional safety check for a
  negative entropy_count -- which should never happen, and to also add
  an additional paranoia check to prevent overly large count values to
  be passed into urandom_read().  ]

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org

drivers/char/random.c

@@ -641,7 +641,7 @@ retry:
 		} while (unlikely(entropy_count < pool_size-2 && pnfrac));
 	}
 
-	if (entropy_count < 0) {
+	if (unlikely(entropy_count < 0)) {
 		pr_warn("random: negative entropy/overflow: pool %s count %d\n",
 			r->name, entropy_count);
 		WARN_ON(1);
@@ -981,7 +981,7 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 		      int reserved)
 {
 	int entropy_count, orig;
-	size_t ibytes;
+	size_t ibytes, nfrac;
 
 	BUG_ON(r->entropy_count > r->poolinfo->poolfracbits);
 
@@ -999,7 +999,17 @@ retry:
 	}
 	if (ibytes < min)
 		ibytes = 0;
-	if ((entropy_count -= ibytes << (ENTROPY_SHIFT + 3)) < 0)
+
+	if (unlikely(entropy_count < 0)) {
+		pr_warn("random: negative entropy count: pool %s count %d\n",
+			r->name, entropy_count);
+		WARN_ON(1);
+		entropy_count = 0;
+	}
+	nfrac = ibytes << (ENTROPY_SHIFT + 3);
+	if ((size_t) entropy_count > nfrac)
+		entropy_count -= nfrac;
+	else
 		entropy_count = 0;
 
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
@@ -1376,6 +1386,7 @@ urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 			    "with %d bits of entropy available\n",
 			    current->comm, nonblocking_pool.entropy_total);
 
+	nbytes = min_t(size_t, nbytes, INT_MAX >> (ENTROPY_SHIFT + 3));
 	ret = extract_entropy_user(&nonblocking_pool, buf, nbytes);
 
 	trace_urandom_read(8 * nbytes, ENTROPY_BITS(&nonblocking_pool),