lkdtm: add "WRITE_KERN" test

Add "WRITE_KERN" crash target to validate that kernel executable memory
is not writable.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
index d6ee1aae3e72..d30301149242 100644
--- a/drivers/misc/lkdtm.c
+++ b/drivers/misc/lkdtm.c
@@ -102,6 +102,7 @@ enum ctype {
         CT_EXEC_USERSPACE,
         CT_ACCESS_USERSPACE,
         CT_WRITE_RO,
+        CT_WRITE_KERN,
 };
 
 static char* cp_name[] = {
@@ -138,6 +139,7 @@ static char* cp_type[] = {
         "EXEC_USERSPACE",
         "ACCESS_USERSPACE",
         "WRITE_RO",
+        "WRITE_KERN",
 };
 
 static struct jprobe lkdtm;
@@ -317,6 +319,13 @@ static void do_nothing(void)
         return;
 }
 
+/* Must immediately follow do_nothing for size calculuations to work out. */
+static void do_overwritten(void)
+{
+        pr_info("do_overwritten wasn't overwritten!\n");
+        return;
+}
+
 static noinline void corrupt_stack(void)
 {
         /* Use default char array length that triggers stack protection. */
@@ -496,6 +505,22 @@ static void lkdtm_do_action(enum ctype which)
 
                 break;
         }
+        case CT_WRITE_KERN: {
+                size_t size;
+                unsigned char *ptr;
+
+                size = (unsigned long)do_overwritten -
+                       (unsigned long)do_nothing;
+                ptr = (unsigned char *)do_overwritten;
+
+                pr_info("attempting bad %zu byte write at %p\n", size, ptr);
+                memcpy(ptr, (unsigned char *)do_nothing, size);
+                flush_icache_range((unsigned long)ptr,
+                                   (unsigned long)(ptr + size));
+
+                do_overwritten();
+                break;
+        }
         case CT_NONE:
         default:
                 break;