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Date: Tue, 9 Jan 2024 16:36:03 +0100
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From: Marco Pagani <marpagan@redhat.com>
Subject: Re: [PATCH v3] kunit: run test suites only after module
 initialization completes
To: David Gow <davidgow@google.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>,
 Shuah Khan <skhan@linuxfoundation.org>, Jinjie Ruan <ruanjinjie@huawei.com>,
 Rae Moar <rmoar@google.com>, Richard Fitzgerald <rf@opensource.cirrus.com>,
 Javier Martinez Canillas <javierm@redhat.com>,
 linux-kselftest@vger.kernel.org, kunit-dev@googlegroups.com,
 linux-kernel@vger.kernel.org
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On 2024-01-08 08:27, David Gow wrote:
> On Wed, 6 Dec 2023 at 23:07, Marco Pagani <marpagan@redhat.com> wrote:
>>
>> Commit 2810c1e99867 ("kunit: Fix wild-memory-access bug in
>> kunit_free_suite_set()") fixed a wild-memory-access bug that could have
>> happened during the loading phase of test suites built and executed as
>> loadable modules. However, it also introduced a problematic side effect
>> that causes test suites modules to crash when they attempt to register
>> fake devices.
>>
>> When a module is loaded, it traverses the MODULE_STATE_UNFORMED and
>> MODULE_STATE_COMING states before reaching the normal operating state
>> MODULE_STATE_LIVE. Finally, when the module is removed, it moves to
>> MODULE_STATE_GOING before being released. However, if the loading
>> function load_module() fails between complete_formation() and
>> do_init_module(), the module goes directly from MODULE_STATE_COMING to
>> MODULE_STATE_GOING without passing through MODULE_STATE_LIVE.
>>
>> This behavior was causing kunit_module_exit() to be called without
>> having first executed kunit_module_init(). Since kunit_module_exit() is
>> responsible for freeing the memory allocated by kunit_module_init()
>> through kunit_filter_suites(), this behavior was resulting in a
>> wild-memory-access bug.
>>
>> Commit 2810c1e99867 ("kunit: Fix wild-memory-access bug in
>> kunit_free_suite_set()") fixed this issue by running the tests when the
>> module is still in MODULE_STATE_COMING. However, modules in that state
>> are not fully initialized, lacking sysfs kobjects. Therefore, if a test
>> module attempts to register a fake device, it will inevitably crash.
>>
>> This patch proposes a different approach to fix the original
>> wild-memory-access bug while restoring the normal module execution flow
>> by making kunit_module_exit() able to detect if kunit_module_init() has
>> previously initialized the tests suite set. In this way, test modules
>> can once again register fake devices without crashing.
>>
>> This behavior is achieved by checking whether mod->kunit_suites is a
>> virtual or direct mapping address. If it is a virtual address, then
>> kunit_module_init() has allocated the suite_set in kunit_filter_suites()
>> using kmalloc_array(). On the contrary, if mod->kunit_suites is still
>> pointing to the original address that was set when looking up the
>> .kunit_test_suites section of the module, then the loading phase has
>> failed and there's no memory to be freed.
>>
>> v3:
>> - add a comment to clarify why the start address is checked
>> v2:
>> - add include <linux/mm.h>
>>
>> Fixes: 2810c1e99867 ("kunit: Fix wild-memory-access bug in kunit_free_suite_set()")
>> Tested-by: Richard Fitzgerald <rf@opensource.cirrus.com>
>> Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
>> Signed-off-by: Marco Pagani <marpagan@redhat.com>
>> ---
> 
> Sorry for the delay here: there are enough subtleties here that I
> wanted to double check some things.
> 
> I keep feeling that there has to be a nicer way of doing this, but I
> can't think of one, so let's go with this, since it's fixing a real
> issue.
> 
> I'm a little hesitant about our use of the suite_set.start address as
> an 'is initialised' flag, and depending on it being reallocated via
> kunit_filter_suites(), but since we already depend on that (by always
> using kunit_free_suite_set()), I'm okay with it.
>

I have the same feeling. I spent some thinking about alternative
solutions that did not require adding a flag in the module struct or
restructuring significant portions of the code, but I could not think of
anything better for the moment.

> My only request (other than this needing a rebase, probably on top of
> 6.8) would be to add a comment in kunit_filter_suites() noting that it
> must return a virtual address. That's probably something we should've
> done a while ago, but I can just see this requirement getting
> forgotten.
> 

Sure, I'll do it.

Thanks,
Marco

> Reviewed-by: David Gow <davidgow@google.com>
> 
> 
>>  lib/kunit/test.c | 14 +++++++++++---
>>  1 file changed, 11 insertions(+), 3 deletions(-)
>>
>> diff --git a/lib/kunit/test.c b/lib/kunit/test.c
>> index 7aceb07a1af9..3263e0d5e0f6 100644
>> --- a/lib/kunit/test.c
>> +++ b/lib/kunit/test.c
>> @@ -16,6 +16,7 @@
>>  #include <linux/panic.h>
>>  #include <linux/sched/debug.h>
>>  #include <linux/sched.h>
>> +#include <linux/mm.h>
>>
>>  #include "debugfs.h"
>>  #include "hooks-impl.h"
>> @@ -775,12 +776,19 @@ static void kunit_module_exit(struct module *mod)
>>         };
>>         const char *action = kunit_action();
>>
>> +       /*
>> +        * Check if the start address is a valid virtual address to detect
>> +        * if the module load sequence has failed and the suite set has not
>> +        * been initialized and filtered.
>> +        */
>> +       if (!suite_set.start || !virt_addr_valid(suite_set.start))
>> +               return;
>> +
>>         if (!action)
>>                 __kunit_test_suites_exit(mod->kunit_suites,
>>                                          mod->num_kunit_suites);
>>
>> -       if (suite_set.start)
>> -               kunit_free_suite_set(suite_set);
>> +       kunit_free_suite_set(suite_set);
>>  }
>>
>>  static int kunit_module_notify(struct notifier_block *nb, unsigned long val,
>> @@ -790,12 +798,12 @@ static int kunit_module_notify(struct notifier_block *nb, unsigned long val,
>>
>>         switch (val) {
>>         case MODULE_STATE_LIVE:
>> +               kunit_module_init(mod);
>>                 break;
>>         case MODULE_STATE_GOING:
>>                 kunit_module_exit(mod);
>>                 break;
>>         case MODULE_STATE_COMING:
>> -               kunit_module_init(mod);
>>                 break;
>>         case MODULE_STATE_UNFORMED:
>>                 break;
>>
>> base-commit: 33cc938e65a98f1d29d0a18403dbbee050dcad9a
>> --
>> 2.43.0
>>