mm: unexport globally copy_to_kernel_nofault

copy_to_kernel_nofault() is an internal helper which should not be visible
to loadable modules – exporting it would give exploit code a cheap
oracle to probe kernel addresses.  Instead, keep the helper un-exported
and compile the kunit case that exercises it only when
mm/kasan/kasan_test.o is linked into vmlinux.

[[email protected]: add a brief comment to `#ifndef MODULE`]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: ca79a00bb9 ("kasan: migrate copy_user_test to kunit")
Signed-off-by: Sabyrzhan Tasbolatov <[email protected]>
Suggested-by: Christoph Hellwig <[email protected]>
Suggested-by: Marco Elver <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Reviewed-by: Andrey Konovalov <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Dmitriy Vyukov <[email protected]>
Cc: Vincenzo Frascino <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/kasan/kasan_test_c.c

@@ -1977,6 +1977,11 @@ static void rust_uaf(struct kunit *test)
 	KUNIT_EXPECT_KASAN_FAIL(test, kasan_test_rust_uaf());
 }
 
+/*
+ * copy_to_kernel_nofault() is an internal helper available when
+ * kasan_test is built-in, so it must not be visible to loadable modules.
+ */
+#ifndef MODULE
 static void copy_to_kernel_nofault_oob(struct kunit *test)
 {
 	char *ptr;
@@ -2011,6 +2016,7 @@ static void copy_to_kernel_nofault_oob(struct kunit *test)
 
 	kfree(ptr);
 }
+#endif /* !MODULE */
 
 static void copy_user_test_oob(struct kunit *test)
 {
@@ -2131,7 +2137,9 @@ static struct kunit_case kasan_kunit_test_cases[] = {
 	KUNIT_CASE(match_all_not_assigned),
 	KUNIT_CASE(match_all_ptr_tag),
 	KUNIT_CASE(match_all_mem_tag),
+#ifndef MODULE
 	KUNIT_CASE(copy_to_kernel_nofault_oob),
+#endif
 	KUNIT_CASE(rust_uaf),
 	KUNIT_CASE(copy_user_test_oob),
 	{}

mm/maccess.c

@@ -82,7 +82,6 @@ Efault:
 	pagefault_enable();
 	return -EFAULT;
 }
-EXPORT_SYMBOL_GPL(copy_to_kernel_nofault);
 
 long strncpy_from_kernel_nofault(char *dst, const void *unsafe_addr, long count)
 {