Calling uname() with the UNAME26 personality set allows a leak of kernel
stack contents. This fixes it by initializing the stack buffer to zero,
defensively calculating the length of copy_to_user() call, and making
the len argument unsigned.

CVE-2012-0957

Reported-by: PaX Team <pageexec@freemail.hu>
Cc: stable@vger.kernel.org
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

---
v2:
 - corrected credit to PaX Team.
 - moved stack clearing into if case to avoid penalty to common case.
---
 kernel/sys.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/kernel/sys.c b/kernel/sys.c
index c5cb5b9..2300248 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1265,13 +1265,13 @@ DECLARE_RWSEM(uts_sem);
  * Work around broken programs that cannot handle "Linux 3.0".
  * Instead we map 3.x to 2.6.40+x, so e.g. 3.0 would be 2.6.40
  */
-static int override_release(char __user *release, int len)
+static int override_release(char __user *release, size_t len)
 {
 	int ret = 0;
-	char buf[65];
 
 	if (current->personality & UNAME26) {
-		char *rest = UTS_RELEASE;
+		const char *rest = UTS_RELEASE;
+		char buf[65] = { 0 };
 		int ndots = 0;
 		unsigned v;
 
@@ -1283,7 +1283,9 @@ static int override_release(char __user *release, int len)
 			rest++;
 		}
 		v = ((LINUX_VERSION_CODE >> 8) & 0xff) + 40;
-		snprintf(buf, len, "2.6.%u%s", v, rest);
+		snprintf(buf, sizeof(buf), "2.6.%u%s", v, rest);
+		if (sizeof(buf) < len)
+			len = sizeof(buf);
 		ret = copy_to_user(release, buf, len);
 	}
 	return ret;
-- 
1.7.9.5

-- 
Kees Cook
Chrome OS Security