lkdtm: Update WRITE_AFTER_FREE test

The SLUB allocator may use the first word of a freed block to store the
freelist information. This may make it harder to test poisoning
features. Change the WRITE_AFTER_FREE test to better match what
the READ_AFTER_FREE test does and also print out a big more information.

Signed-off-by: Laura Abbott <labbott@fedoraproject.org>
Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
index 8de4746..a00a2b1 100644 (file)
--- a/drivers/misc/lkdtm.c
+++ b/drivers/misc/lkdtm.c
@@ -411,12 +411,21 @@ static void lkdtm_do_action(enum ctype which)
                break;
        }
        case CT_WRITE_AFTER_FREE: {
+               int *base;
                size_t len = 1024;
-               u32 *data = kmalloc(len, GFP_KERNEL);
+               /*
+                * The slub allocator uses the first word to store the free
+                * pointer in some configurations. Use the middle of the
+                * allocation to avoid running into the freelist
+                */
+               size_t offset = (len / sizeof(*base)) / 2;
 
-               kfree(data);
-               schedule();
-               memset(data, 0x78, len);
+               base = kmalloc(len, GFP_KERNEL);
+               pr_info("Allocated memory %p-%p\n", base, &base[offset * 2]);
+               kfree(base);
+               pr_info("Attempting bad write to freed memory at %p\n",
+                       &base[offset]);
+               base[offset] = 0x0abcdef0;
                break;
        }
        case CT_READ_AFTER_FREE: {