test: check copy_to/from_user boundary validation

To help avoid an architecture failing to correctly check kernel/user
boundaries when handling copy_to_user, copy_from_user, put_user, or
get_user, perform some simple tests and fail to load if any of them
behave unexpectedly.

Specifically, this is to make sure there is a way to notice if things
like what was fixed in commit 8404663f81 ("ARM: 7527/1: uaccess:
explicitly check __user pointer when !CPU_USE_DOMAINS") ever regresses
again, for any architecture.

Additionally, adds new "user" selftest target, which loads this module.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/Makefile

@@ -9,6 +9,7 @@ TARGETS += ptrace
 TARGETS += timers
 TARGETS += vm
 TARGETS += powerpc
+TARGETS += user
 
 all:
 	for TARGET in $(TARGETS); do \

tools/testing/selftests/user/Makefile

@@ -0,0 +1,13 @@
+# Makefile for user memory selftests
+
+# No binaries, but make sure arg-less "make" doesn't trigger "run_tests"
+all:
+
+run_tests: all
+	@if /sbin/modprobe test_user_copy ; then \
+		rmmod test_user_copy; \
+		echo "user_copy: ok"; \
+	else \
+		echo "user_copy: [FAIL]"; \
+		exit 1; \
+	fi