- Add new CORRUPT_STACK_STRONG to test -fstack-protector-strong, since

the existing CORRUPT_STACK test only tested regular -fstack-protector.
 - Add pair of tests for checking kernel stack leading/trailing guard pages
   under VMAP_STACK: STACK_GUARD_PAGE_LEADING and STACK_GUARD_PAGE_TRAILING.
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 Comment: Kees Cook <kees@outflux.net>
 
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Merge tag 'lkdtm-next-part2' of git://git.kernel.org/pub/scm/linux/kernel/git/kees/linux into char-misc-next

Kees writes:

- Add new CORRUPT_STACK_STRONG to test -fstack-protector-strong, since
  the existing CORRUPT_STACK test only tested regular -fstack-protector.
- Add pair of tests for checking kernel stack leading/trailing guard pages
  under VMAP_STACK: STACK_GUARD_PAGE_LEADING and STACK_GUARD_PAGE_TRAILING.

drivers/misc/lkdtm.h

@@ -14,6 +14,7 @@ void lkdtm_EXCEPTION(void);
 void lkdtm_LOOP(void);
 void lkdtm_OVERFLOW(void);
 void lkdtm_CORRUPT_STACK(void);
+void lkdtm_CORRUPT_STACK_STRONG(void);
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void);
 void lkdtm_SOFTLOCKUP(void);
 void lkdtm_HARDLOCKUP(void);
@@ -22,6 +23,8 @@ 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 {
@@ -84,16 +85,31 @@ void lkdtm_OVERFLOW(void)
 
 static noinline void __lkdtm_CORRUPT_STACK(void *stack)
 {
-	memset(stack, 'a', 64);
+	memset(stack, '\xff', 64);
 }
 
+/* This should trip the stack canary, not corrupt the return address. */
 noinline void lkdtm_CORRUPT_STACK(void)
 {
 	/* Use default char array length that triggers stack protection. */
-	char data[8];
+	char data[8] __aligned(sizeof(void *));
+
 	__lkdtm_CORRUPT_STACK(&data);
 
-	pr_info("Corrupted stack with '%16s'...\n", data);
+	pr_info("Corrupted stack containing char array ...\n");
+}
+
+/* Same as above but will only get a canary with -fstack-protector-strong */
+noinline void lkdtm_CORRUPT_STACK_STRONG(void)
+{
+	union {
+		unsigned short shorts[4];
+		unsigned long *ptr;
+	} data __aligned(sizeof(void *));
+
+	__lkdtm_CORRUPT_STACK(&data);
+
+	pr_info("Corrupted stack containing union ...\n");
 }
 
 void lkdtm_UNALIGNED_LOAD_STORE_WRITE(void)
@@ -199,6 +215,7 @@ 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 +224,31 @@ 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 +201,9 @@ struct crashtype crashtypes[] = {
 	CRASHTYPE(CORRUPT_LIST_DEL),
 	CRASHTYPE(CORRUPT_USER_DS),
 	CRASHTYPE(CORRUPT_STACK),
+	CRASHTYPE(CORRUPT_STACK_STRONG),
+	CRASHTYPE(STACK_GUARD_PAGE_LEADING),
+	CRASHTYPE(STACK_GUARD_PAGE_TRAILING),
 	CRASHTYPE(UNALIGNED_LOAD_STORE_WRITE),
 	CRASHTYPE(OVERWRITE_ALLOCATION),
 	CRASHTYPE(WRITE_AFTER_FREE),