[PATCH v2] mm: unexport globally copy_to_kernel_nofault

[Sabyrzhan Tasbolatov <snovitoll@gmail.com>]

`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.

Fixes: ca79a00bb9a8 ("kasan: migrate copy_user_test to kunit")
Suggested-by: Christoph Hellwig <hch@infradead.org>
Suggested-by: Marco Elver <elver@google.com>
Reviewed-by: Andrey Konovalov <andreyknvl@gmail.com>
Signed-off-by: Sabyrzhan Tasbolatov <snovitoll@gmail.com>
---
Changes v2:
- add a brief comment to `#ifndef MODULE`
---
 mm/kasan/kasan_test_c.c | 8 ++++++++
 mm/maccess.c            | 1 -
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/kasan_test_c.c b/mm/kasan/kasan_test_c.c
index 5f922dd38ffa..2aa12dfa427a 100644
--- a/mm/kasan/kasan_test_c.c
+++ b/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),
 	{}
diff --git a/mm/maccess.c b/mm/maccess.c
index 831b4dd7296c..486559d68858 100644
--- a/mm/maccess.c
+++ b/mm/maccess.c
@@ -82,7 +82,6 @@ long copy_to_kernel_nofault(void *dst, const void *src, size_t size)
 	pagefault_enable();
 	return -EFAULT;
 }
-EXPORT_SYMBOL_GPL(copy_to_kernel_nofault);
 
 long strncpy_from_kernel_nofault(char *dst, const void *unsafe_addr, long count)
 {
-- 
2.34.1