[PATCH v3] mm/kfence: Add a new kunit test test_use_after_free_read_nofault()

[Ritesh Harjani (IBM)]

From: Nirjhar Roy <nirjhar at linux.ibm.com>

Faults from copy_from_kernel_nofault() needs 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.

Co-developed-by: Ritesh Harjani (IBM) <ritesh.list at gmail.com>
Signed-off-by: Nirjhar Roy <nirjhar at linux.ibm.com>
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list at gmail.com>
---

Will be nice if we can get some feedback on this.

v2 -> v3:
=========
1. Separated out this kfence kunit test from the larger powerpc+kfence+v3 series.
2. Dropped RFC tag

[v2]: https://lore.kernel.org/linuxppc-dev/cover.1728954719.git.ritesh.list@gmail.com
[powerpc+kfence+v3]: https://lore.kernel.org/linuxppc-dev/cover.1729271995.git.ritesh.list@gmail.com

 mm/kfence/kfence_test.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/mm/kfence/kfence_test.c b/mm/kfence/kfence_test.c
index 00fd17285285..f65fb182466d 100644
--- a/mm/kfence/kfence_test.c
+++ b/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),
--
2.46.0