linux-kernel - [PATCH] kunit: Fix race condition in try-catch completion

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Message-ID: <20240412025903.489470-1-davidgow@google.com>
Date: Fri, 12 Apr 2024 10:59:01 +0800
From: David Gow <davidgow@...gle.com>
To: Rae Moar <rmoar@...gle.com>, Kees Cook <keescook@...omium.org>, 
	"Mickaël Salaün" <mic@...ikod.net>, Naresh Kamboju <naresh.kamboju@...aro.org>, 
	Shuah Khan <skhan@...uxfoundation.org>
Cc: David Gow <davidgow@...gle.com>, Will Deacon <will@...nel.org>, 
	Dan Carpenter <dan.carpenter@...aro.org>, Guenter Roeck <linux@...ck-us.net>, 
	linux-kselftest@...r.kernel.org, kunit-dev@...glegroups.com, 
	linux-kernel@...r.kernel.org, Brendan Higgins <brendan.higgins@...ux.dev>, 
	Linux Kernel Functional Testing <lkft@...aro.org>
Subject: [PATCH] kunit: Fix race condition in try-catch completion

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@...aro.org>
Closes: https://lore.kernel.org/lkml/20240410102710.35911-1-naresh.kamboju@linaro.org/
Tested-by: Linux Kernel Functional Testing <lkft@...aro.org>
Acked-by: Mickaël Salaün <mic@...ikod.net>
Signed-off-by: David Gow <davidgow@...gle.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_catch, void *context)
 {
 	struct kunit *test = try_catch->test;
 	struct task_struct *task_struct;
+	struct completion *task_done;
 	int exit_code, time_remaining;
 
 	try_catch->context = 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 = wait_for_completion_timeout(task_struct->vfork_done,
+	task_done = task_struct->vfork_done;
+	wake_up_process(task_struct);
+
+	time_remaining = wait_for_completion_timeout(task_done,
 						     kunit_test_timeout());
 	if (time_remaining == 0) {
 		try_catch->try_result = -ETIMEDOUT;
-- 
2.44.0.683.g7961c838ac-goog