kernel/sys.c: fix stack memory content leak via UNAME26

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

CVE-2012-0957

Reported-by: PaX Team <pageexec@freemail.hu>
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: PaX Team <pageexec@freemail.hu>
Cc: Brad Spengler <spender@grsecurity.net>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 7 insertions, 5 deletions

diff --git a/kernel/sys.c b/kernel/sys.c
index c5cb5b99cb81..01865c6fb6a0 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1265,15 +1265,16 @@ 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;
+                size_t copy;
 
                 while (*rest) {
                         if (*rest == '.' && ++ndots >= 3)
@@ -1283,8 +1284,9 @@ 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);
-                ret = copy_to_user(release, buf, len);
+                copy = min(sizeof(buf), max_t(size_t, 1, len));
+                copy = scnprintf(buf, copy, "2.6.%u%s", v, rest);
+                ret = copy_to_user(release, buf, copy + 1);
         }
         return ret;
 }