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

1 files changed, 14 insertions, 3 deletions

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 0a7ac0a7b252..71529e196b84 100644
--- a/drivers/char/random.c
+++ b/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),