lkdtm: add bad USER_DS test

This adds CORRUPT_USER_DS to check that the get_fs() test on syscall
return (via __VERIFY_PRE_USERMODE_STATE) still sees USER_DS. Since
trying to deal with values other than USER_DS and KERNEL_DS across all
architectures in a safe way is not sensible, this sets KERNEL_DS, but
since that could be extremely dangerous if the protection is not present,
it also raises SIGKILL for current, so that no matter what, the process
will die. A successful test will be visible with a BUG(), like all the
other LKDTM tests.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm.h

@@ -27,6 +27,7 @@ void lkdtm_REFCOUNT_ZERO_SUB(void);
 void lkdtm_REFCOUNT_ZERO_ADD(void);
 void lkdtm_CORRUPT_LIST_ADD(void);
 void lkdtm_CORRUPT_LIST_DEL(void);
+void lkdtm_CORRUPT_USER_DS(void);
 
 /* lkdtm_heap.c */
 void lkdtm_OVERWRITE_ALLOCATION(void);

drivers/misc/lkdtm_bugs.c

@@ -8,6 +8,8 @@
 #include <linux/list.h>
 #include <linux/refcount.h>
 #include <linux/sched.h>
+#include <linux/sched/signal.h>
+#include <linux/uaccess.h>
 
 struct lkdtm_list {
 	struct list_head node;
@@ -279,3 +281,12 @@ void lkdtm_CORRUPT_LIST_DEL(void)
 	else
 		pr_err("list_del() corruption not detected!\n");
 }
+
+void lkdtm_CORRUPT_USER_DS(void)
+{
+	pr_info("setting bad task size limit\n");
+	set_fs(KERNEL_DS);
+
+	/* Make sure we do not keep running with a KERNEL_DS! */
+	force_sig(SIGKILL, current);
+}

drivers/misc/lkdtm_core.c

@@ -199,6 +199,7 @@ struct crashtype crashtypes[] = {
 	CRASHTYPE(OVERFLOW),
 	CRASHTYPE(CORRUPT_LIST_ADD),
 	CRASHTYPE(CORRUPT_LIST_DEL),
+	CRASHTYPE(CORRUPT_USER_DS),
 	CRASHTYPE(CORRUPT_STACK),
 	CRASHTYPE(UNALIGNED_LOAD_STORE_WRITE),
 	CRASHTYPE(OVERWRITE_ALLOCATION),