linux-kernel - [PATCH 4/4] kunit: make knuit_kfree() not segfault on invalid inputs

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Message-Id: <20220721180214.3223778-4-dlatypov@google.com>
Date:   Thu, 21 Jul 2022 18:02:14 +0000
From:   Daniel Latypov <dlatypov@...gle.com>
To:     brendanhiggins@...gle.com, davidgow@...gle.com
Cc:     linux-kernel@...r.kernel.org, kunit-dev@...glegroups.com,
        linux-kselftest@...r.kernel.org, skhan@...uxfoundation.org,
        Daniel Latypov <dlatypov@...gle.com>
Subject: [PATCH 4/4] kunit: make knuit_kfree() not segfault on invalid inputs

kunit_kfree() can only work on data ("resources") allocated by KUnit.

Currently for code like this,
> void *ptr = kmalloc(4, GFP_KERNEL);
> kunit_kfree(test, ptr);
kunit_kfree() will segfault.

It'll try and look up the kunit_resource associated with `ptr` and get a
NULL back, but it won't check for this. This means we also segfault if
you double-free.

Change kunit_kfree() so it'll notice these invalid pointers and respond
by failing the test.

Implementation: kunit_destroy_resource() does what kunit_kfree() does,
but is more generic and returns -ENOENT when it can't find the resource.
Sadly, unlike just letting it crash, this means we don't get a stack
trace. But kunit_kfree() is so infrequently used it shouldn't be hard to
track down the bad callsite anyways.

After this change, the above code gives:
> # example_simple_test: EXPECTATION FAILED at lib/kunit/test.c:702
> kunit_kfree: 00000000626ec200 already freed or not allocated by kunit

Signed-off-by: Daniel Latypov <dlatypov@...gle.com>
---
 lib/kunit/test.c | 14 ++------------
 1 file changed, 2 insertions(+), 12 deletions(-)

diff --git a/lib/kunit/test.c b/lib/kunit/test.c
index 82019a78462e..c7ca87484968 100644
--- a/lib/kunit/test.c
+++ b/lib/kunit/test.c
@@ -698,18 +698,8 @@ static inline bool kunit_kfree_match(struct kunit *test,
 
 void kunit_kfree(struct kunit *test, const void *ptr)
 {
-	struct kunit_resource *res;
-
-	res = kunit_find_resource(test, kunit_kfree_match, (void *)ptr);
-
-	/*
-	 * Removing the resource from the list of resources drops the
-	 * reference count to 1; the final put will trigger the free.
-	 */
-	kunit_remove_resource(test, res);
-
-	kunit_put_resource(res);
-
+	if (kunit_destroy_resource(test, kunit_kfree_match, (void *)ptr))
+		KUNIT_FAIL(test, "kunit_kfree: %px already freed or not allocated by kunit", ptr);
 }
 EXPORT_SYMBOL_GPL(kunit_kfree);
 
-- 
2.37.1.359.gd136c6c3e2-goog