lkdtm: fix ACCESS_USERSPACE test

Add a copy_to_user() call to the ACCESS_USERSPACE test
prior to attempting direct dereferencing of the user
address to ensure the page is present.  Otherwise,
a fault occurs on arm kernels even prior to the introduction
of CONFIG_CPU_SW_DOMAIN_PAN, and there is no difference in
behavior for CONFIG_CPU_SW_DOMAIN_PAN=n vs CONFIG_CPU_SW_DOMAIN_PAN=y.

Before this change, for any value of CONFIG_CPU_SW_DOMAIN_PAN:
lkdtm: Performing direct entry ACCESS_USERSPACE
lkdtm: attempting bad read at b6fe8000
Unable to handle kernel paging request at virtual address b6fe8000

After this change, for CONFIG_CPU_SW_DOMAIN_PAN=n:
lkdtm: Performing direct entry ACCESS_USERSPACE
lkdtm: attempting bad read at b6efc000
lkdtm: attempting bad write at b6efc000

After this change, for CONFIG_CPU_SW_DOMAIN_PAN=y:
lkdtm: Performing direct entry ACCESS_USERSPACE
lkdtm: attempting bad read at b6f7d000
Unhandled fault: page domain fault (0x01b) at 0xb6f7d000
...
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/lkdtm.c

@@ -472,7 +472,7 @@ static void lkdtm_do_action(enum ctype which)
 		break;
 	}
 	case CT_ACCESS_USERSPACE: {
-		unsigned long user_addr, tmp;
+		unsigned long user_addr, tmp = 0;
 		unsigned long *ptr;
 
 		user_addr = vm_mmap(NULL, 0, PAGE_SIZE,
@@ -483,6 +483,12 @@ static void lkdtm_do_action(enum ctype which)
 			return;
 		}
 
+		if (copy_to_user((void __user *)user_addr, &tmp, sizeof(tmp))) {
+			pr_warn("copy_to_user failed\n");
+			vm_munmap(user_addr, PAGE_SIZE);
+			return;
+		}
+
 		ptr = (unsigned long *)user_addr;
 
 		pr_info("attempting bad read at %p\n", ptr);