[PATCH 3.15 094/109] random: check for increase of entropy_count because of signed conversion

Greg Kroah-Hartman
2014-07-26T19:24:42Z

From: Hannes Frederic Sowa <hannes@stressinduktion.org>

commit 79a8468747c5f95ed3d5ce8376a3e82e0c5857fc upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/char/random.c | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)

--- 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);
@@ -980,7 +980,7 @@ static size_t account(struct entropy_sto
int reserved)
{
int entropy_count, orig;
- size_t ibytes;
+ size_t ibytes, nfrac;

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

@@ -998,7 +998,17 @@ 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)
@@ -1375,6 +1385,7 @@ urandom_read(struct file *file, char __u
"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),