From: Theodore Ts'o <tytso@mit.edu>
Date: Sat, 17 May 2014 01:40:41 +0000 (-0400)
Subject: random: fix BUG_ON caused by accounting simplification
X-Git-Tag: android-x86-4.4-r2~1132^2
X-Git-Url: http://git.osdn.net/view?a=commitdiff_plain;h=f9c6d4987b23e0a514464bae6771933a48e4cd01;p=android-x86%2Fkernel.git

random: fix BUG_ON caused by accounting simplification

Commit ee1de406ba6eb1 ("random: simplify accounting logic") simplified
things too much, in that it allows the following to trigger an
overflow that results in a BUG_ON crash:

dd if=/dev/urandom of=/dev/zero bs=67108707 count=1

Thanks to Peter Zihlstra for discovering the crash, and Hannes
Frederic for analyizing the root cause.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Reported-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Greg Price <price@mit.edu>
---

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 6b75713d953a..102c50d38902 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -995,8 +995,11 @@ retry:
 		ibytes = min_t(size_t, ibytes, have_bytes - reserved);
 	if (ibytes < min)
 		ibytes = 0;
-	entropy_count = max_t(int, 0,
-			      entropy_count - (ibytes << (ENTROPY_SHIFT + 3)));
+	if (have_bytes >= ibytes + reserved)
+		entropy_count -= ibytes << (ENTROPY_SHIFT + 3);
+	else
+		entropy_count = reserved << (ENTROPY_SHIFT + 3);
+
 	if (cmpxchg(&r->entropy_count, orig, entropy_count) != orig)
 		goto retry;