lkdtm: improve use-after-free tests

This improves the order of operations on the use-after-free tests to
try to make sure we've executed any available sanity-checking code,
and to report the poisoning that was found.

Signed-off-by: Kees Cook <keescook@chromium.org>

diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
index c333e81..9345999 100644 (file)
--- a/drivers/misc/lkdtm.c
+++ b/drivers/misc/lkdtm.c
@@ -417,7 +417,7 @@ static void lkdtm_do_action(enum ctype which)
                break;
        }
        case CT_WRITE_AFTER_FREE: {
-               int *base;
+               int *base, *again;
                size_t len = 1024;
                /*
                 * The slub allocator uses the first word to store the free
@@ -428,10 +428,16 @@ static void lkdtm_do_action(enum ctype which)
 
                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]);
+               kfree(base);
                base[offset] = 0x0abcdef0;
+               /* Attempt to notice the overwrite. */
+               again = kmalloc(len, GFP_KERNEL);
+               kfree(again);
+               if (again != base)
+                       pr_info("Hmm, didn't get the same memory range.\n");
+
                break;
        }
        case CT_READ_AFTER_FREE: {
@@ -462,7 +468,7 @@ static void lkdtm_do_action(enum ctype which)
                saw = base[offset];
                if (saw != *val) {
                        /* Good! Poisoning happened, so declare a win. */
-                       pr_info("Memory correctly poisoned, calling BUG\n");
+                       pr_info("Memory correctly poisoned (%x)\n", saw);
                        BUG();
                }
                pr_info("Memory was not poisoned\n");
@@ -480,6 +486,11 @@ static void lkdtm_do_action(enum ctype which)
                schedule();
                pr_info("Attempting bad write to the buddy page after free\n");
                memset((void *)p, 0x78, PAGE_SIZE);
+               /* Attempt to notice the overwrite. */
+               p = __get_free_page(GFP_KERNEL);
+               free_page(p);
+               schedule();
+
                break;
        }
        case CT_READ_BUDDY_AFTER_FREE: {
@@ -503,7 +514,7 @@ static void lkdtm_do_action(enum ctype which)
                saw = base[0];
                if (saw != *val) {
                        /* Good! Poisoning happened, so declare a win. */
-                       pr_info("Buddy page correctly poisoned, calling BUG\n");
+                       pr_info("Memory correctly poisoned (%x)\n", saw);
                        BUG();
                }
                pr_info("Buddy page was not poisoned\n");