lkdtm: Test VMAP_STACK allocates leading/trailing guard pages

Two new tests STACK_GUARD_PAGE_LEADING and STACK_GUARD_PAGE_TRAILING
attempt to read the byte before and after, respectively, of the current
stack frame, which should fault.
Signed-off-by: Kees Cook <keescook@chromium.org>

drivers/misc/lkdtm.h

@@ -22,6 +22,8 @@ void lkdtm_HUNG_TASK(void);
 void lkdtm_CORRUPT_LIST_ADD(void);
 void lkdtm_CORRUPT_LIST_DEL(void);
 void lkdtm_CORRUPT_USER_DS(void);
+void lkdtm_STACK_GUARD_PAGE_LEADING(void);
+void lkdtm_STACK_GUARD_PAGE_TRAILING(void);
 
 /* lkdtm_heap.c */
 void lkdtm_OVERWRITE_ALLOCATION(void);

drivers/misc/lkdtm_bugs.c

@@ -8,6 +8,7 @@
 #include <linux/list.h>
 #include <linux/sched.h>
 #include <linux/sched/signal.h>
+#include <linux/sched/task_stack.h>
 #include <linux/uaccess.h>
 
 struct lkdtm_list {
@@ -199,6 +200,7 @@ void lkdtm_CORRUPT_LIST_DEL(void)
 		pr_err("list_del() corruption not detected!\n");
 }
 
+/* Test if unbalanced set_fs(KERNEL_DS)/set_fs(USER_DS) check exists. */
 void lkdtm_CORRUPT_USER_DS(void)
 {
 	pr_info("setting bad task size limit\n");
@@ -207,3 +209,31 @@ void lkdtm_CORRUPT_USER_DS(void)
 	/* Make sure we do not keep running with a KERNEL_DS! */
 	force_sig(SIGKILL, current);
 }
+
+/* Test that VMAP_STACK is actually allocating with a leading guard page */
+void lkdtm_STACK_GUARD_PAGE_LEADING(void)
+{
+	const unsigned char *stack = task_stack_page(current);
+	const unsigned char *ptr = stack - 1;
+	volatile unsigned char byte;
+
+	pr_info("attempting bad read from page below current stack\n");
+
+	byte = *ptr;
+
+	pr_err("FAIL: accessed page before stack!\n");
+}
+
+/* Test that VMAP_STACK is actually allocating with a trailing guard page */
+void lkdtm_STACK_GUARD_PAGE_TRAILING(void)
+{
+	const unsigned char *stack = task_stack_page(current);
+	const unsigned char *ptr = stack + THREAD_SIZE;
+	volatile unsigned char byte;
+
+	pr_info("attempting bad read from page above current stack\n");
+
+	byte = *ptr;
+
+	pr_err("FAIL: accessed page after stack!\n");
+}

drivers/misc/lkdtm_core.c

@@ -201,6 +201,8 @@ struct crashtype crashtypes[] = {
 	CRASHTYPE(CORRUPT_LIST_DEL),
 	CRASHTYPE(CORRUPT_USER_DS),
 	CRASHTYPE(CORRUPT_STACK),
+	CRASHTYPE(STACK_GUARD_PAGE_LEADING),
+	CRASHTYPE(STACK_GUARD_PAGE_TRAILING),
 	CRASHTYPE(UNALIGNED_LOAD_STORE_WRITE),
 	CRASHTYPE(OVERWRITE_ALLOCATION),
 	CRASHTYPE(WRITE_AFTER_FREE),