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From: Rae Moar <rmoar@google.com>
Date: Fri, 12 Apr 2024 17:32:21 -0400
Message-ID: <CA+GJov7BGQmPUCzA_3Em6P0tXAWnUX-YD5r3pdPKFSSNL0tgyw@mail.gmail.com>
Subject: Re: [PATCH] kunit: Fix race condition in try-catch completion
To: David Gow <davidgow@google.com>
Cc: Kees Cook <keescook@chromium.org>, =?UTF-8?B?TWlja2HDq2wgU2FsYcO8bg==?= <mic@digikod.net>, 
	Naresh Kamboju <naresh.kamboju@linaro.org>, Shuah Khan <skhan@linuxfoundation.org>, 
	Will Deacon <will@kernel.org>, Dan Carpenter <dan.carpenter@linaro.org>, 
	Guenter Roeck <linux@roeck-us.net>, linux-kselftest@vger.kernel.org, 
	kunit-dev@googlegroups.com, linux-kernel@vger.kernel.org, 
	Brendan Higgins <brendan.higgins@linux.dev>, 
	Linux Kernel Functional Testing <lkft@linaro.org>
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On Thu, Apr 11, 2024 at 10:59=E2=80=AFPM David Gow <davidgow@google.com> wr=
ote:
>
> KUnit's try-catch infrastructure now uses vfork_done, which is always
> set to a valid completion when a kthread is created, but which is set to
> NULL once the thread terminates. This creates a race condition, where
> the kthread exits before we can wait on it.
>
> Keep a copy of vfork_done, which is taken before we wake_up_process()
> and so valid, and wait on that instead.
>
> Fixes: 4de2a8e4cca4 ("kunit: Handle test faults")
> Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Closes: https://lore.kernel.org/lkml/20240410102710.35911-1-naresh.kamboj=
u@linaro.org/
> Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
> Acked-by: Micka=C3=ABl Sala=C3=BCn <mic@digikod.net>
> Signed-off-by: David Gow <davidgow@google.com>

Hello,

This fix looks good to me. I have tested it and besides the fortify
test error discussed in the previous patch series I am happy.

Thanks!
-Rae

Reviewed-by: Rae Moar <rmoar@google.com>


> ---
>  lib/kunit/try-catch.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/lib/kunit/try-catch.c b/lib/kunit/try-catch.c
> index fa687278ccc9..6bbe0025b079 100644
> --- a/lib/kunit/try-catch.c
> +++ b/lib/kunit/try-catch.c
> @@ -63,6 +63,7 @@ void kunit_try_catch_run(struct kunit_try_catch *try_ca=
tch, void *context)
>  {
>         struct kunit *test =3D try_catch->test;
>         struct task_struct *task_struct;
> +       struct completion *task_done;
>         int exit_code, time_remaining;
>
>         try_catch->context =3D context;
> @@ -75,13 +76,16 @@ void kunit_try_catch_run(struct kunit_try_catch *try_=
catch, void *context)
>                 return;
>         }
>         get_task_struct(task_struct);
> -       wake_up_process(task_struct);
>         /*
>          * As for a vfork(2), task_struct->vfork_done (pointing to the
>          * underlying kthread->exited) can be used to wait for the end of=
 a
> -        * kernel thread.
> +        * kernel thread. It is set to NULL when the thread exits, so we
> +        * keep a copy here.
>          */
> -       time_remaining =3D wait_for_completion_timeout(task_struct->vfork=
_done,
> +       task_done =3D task_struct->vfork_done;
> +       wake_up_process(task_struct);
> +
> +       time_remaining =3D wait_for_completion_timeout(task_done,
>                                                      kunit_test_timeout()=
);
>         if (time_remaining =3D=3D 0) {
>                 try_catch->try_result =3D -ETIMEDOUT;
> --
> 2.44.0.683.g7961c838ac-goog
>