lkdtm: add test for atomic_t underflow/overflow

dmesg output of running this LKDTM test with PaX:

[187095.475573] lkdtm: No crash points registered, enable through debugfs
[187118.020257] lkdtm: Performing direct entry WRAP_ATOMIC
[187118.030045] lkdtm: attempting atomic underflow
[187118.030929] PAX: refcount overflow detected in: bash:1790, uid/euid: 0/0
[187118.071667] PAX: refcount overflow occured at: lkdtm_do_action+0x19e/0x400 [lkdtm]
[187118.081423] CPU: 3 PID: 1790 Comm: bash Not tainted 4.2.6-pax-refcount-split+ #2
[187118.083403] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[187118.102596] task: ffff8800da8de040 ti: ffff8800da8e4000 task.ti: ffff8800da8e4000
[187118.111321] RIP: 0010:[<ffffffffc00fc2fe>]  [<ffffffffc00fc2fe>] lkdtm_do_action+0x19e/0x400 [lkdtm]
...
[187118.128074] lkdtm: attempting atomic overflow
[187118.128080] PAX: refcount overflow detected in: bash:1790, uid/euid: 0/0
[187118.128082] PAX: refcount overflow occured at: lkdtm_do_action+0x1b6/0x400 [lkdtm]
[187118.128085] CPU: 3 PID: 1790 Comm: bash Not tainted 4.2.6-pax-refcount-split+ #2
[187118.128086] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[187118.128088] task: ffff8800da8de040 ti: ffff8800da8e4000 task.ti: ffff8800da8e4000
[187118.128092] RIP: 0010:[<ffffffffc00fc316>]  [<ffffffffc00fc316>] lkdtm_do_action+0x1b6/0x400 [lkdtm]

Signed-off-by: David Windsor <dave@progbits.org>
[cleaned up whitespacing, keescook]
Signed-off-by: Kees Cook <keescook@chromium.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
index 8e00e2e992d1..c333e813ed34 100644
--- a/drivers/misc/lkdtm.c
+++ b/drivers/misc/lkdtm.c
@@ -107,6 +107,7 @@ enum ctype {
         CT_ACCESS_USERSPACE,
         CT_WRITE_RO,
         CT_WRITE_KERN,
+        CT_WRAP_ATOMIC
 };
 
 static char* cp_name[] = {
@@ -147,6 +148,7 @@ static char* cp_type[] = {
         "ACCESS_USERSPACE",
         "WRITE_RO",
         "WRITE_KERN",
+        "WRAP_ATOMIC"
 };
 
 static struct jprobe lkdtm;
@@ -620,6 +622,17 @@ static void lkdtm_do_action(enum ctype which)
                 do_overwritten();
                 break;
         }
+        case CT_WRAP_ATOMIC: {
+                atomic_t under = ATOMIC_INIT(INT_MIN);
+                atomic_t over = ATOMIC_INIT(INT_MAX);
+
+                pr_info("attempting atomic underflow\n");
+                atomic_dec(&under);
+                pr_info("attempting atomic overflow\n");
+                atomic_inc(&over);
+
+                return;
+        }
         case CT_NONE:
         default:
                 break;