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>

1 files changed, 15 insertions, 4 deletions

diff --git a/drivers/misc/lkdtm.c b/drivers/misc/lkdtm.c
index c333e813ed34..9345999f5673 100644
--- 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");