mm/kfence: add a new kunit test test_use_after_free_read_nofault()

Faults from copy_from_kernel_nofault() need to be handled by fixup table
and should not be handled by kfence.  Otherwise while reading /proc/kcore
which uses copy_from_kernel_nofault(), kfence can generate false
negatives.  This can happen when /proc/kcore ends up reading an unmapped
address from kfence pool.

Let's add a testcase to cover this case.

Link: https://lkml.kernel.org/r/210e561f7845697a32de44b643393890f180069f.1729272697.git.ritesh.list@gmail.com
Signed-off-by: Nirjhar Roy <nirjhar@linux.ibm.com>
Co-developed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Tested-by: Marco Elver <elver@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/kfence/kfence_test.c

@@ -383,6 +383,22 @@ static void test_use_after_free_read(struct kunit *test)
 	KUNIT_EXPECT_TRUE(test, report_matches(&expect));
 }
 
+static void test_use_after_free_read_nofault(struct kunit *test)
+{
+	const size_t size = 32;
+	char *addr;
+	char dst;
+	int ret;
+
+	setup_test_cache(test, size, 0, NULL);
+	addr = test_alloc(test, size, GFP_KERNEL, ALLOCATE_ANY);
+	test_free(addr);
+	/* Use after free with *_nofault() */
+	ret = copy_from_kernel_nofault(&dst, addr, 1);
+	KUNIT_EXPECT_EQ(test, ret, -EFAULT);
+	KUNIT_EXPECT_FALSE(test, report_available());
+}
+
 static void test_double_free(struct kunit *test)
 {
 	const size_t size = 32;
@@ -780,6 +796,7 @@ static struct kunit_case kfence_test_cases[] = {
 	KFENCE_KUNIT_CASE(test_out_of_bounds_read),
 	KFENCE_KUNIT_CASE(test_out_of_bounds_write),
 	KFENCE_KUNIT_CASE(test_use_after_free_read),
+	KFENCE_KUNIT_CASE(test_use_after_free_read_nofault),
 	KFENCE_KUNIT_CASE(test_double_free),
 	KFENCE_KUNIT_CASE(test_invalid_addr_free),
 	KFENCE_KUNIT_CASE(test_corruption),