[dpdk-dev] [PATCH v3] eal: fix race in ctrl thread creation

[Luc Pelletier]

The creation of control threads used a pthread barrier for
synchronization. This patch fixes a race condition where the pthread
barrier could get destroyed while one of the threads has not yet
returned from the pthread_barrier_wait function, which could result in
undefined behaviour. The barrier has been completely removed in favour
of a reference count on the control thread parameters struct.

Fixes: 3a0d465d4c53 ("eal: fix use-after-free on control thread creation")
Cc: jianfeng.tan at intel.com
Cc: stable at dpdk.org

Signed-off-by: Luc Pelletier <lucp.at.work at gmail.com>
---

Hi Olivier,
Hi Honnappa,

Thanks for your input Honnappa. I've made the changes to completely
remove the barrier. However, I didn't move the call to
pthread_setaffinity_np to the control thread; I think we want to report
the result of that function to the caller of rte_ctrl_thread_create and
doing so from ctrl_thread_init would be a lot trickier.

Olivier, what do you think of these changes?

 lib/librte_eal/common/eal_common_thread.c | 35 ++++++++---------------
 1 file changed, 12 insertions(+), 23 deletions(-)

diff --git a/lib/librte_eal/common/eal_common_thread.c b/lib/librte_eal/common/eal_common_thread.c
index 73a055902..2421066f9 100644
--- a/lib/librte_eal/common/eal_common_thread.c
+++ b/lib/librte_eal/common/eal_common_thread.c
@@ -169,12 +169,11 @@ __rte_thread_uninit(void)
 struct rte_thread_ctrl_params {
 	void *(*start_routine)(void *);
 	void *arg;
-	pthread_barrier_t configured;
+	unsigned int refcnt;
 };
 
 static void *ctrl_thread_init(void *arg)
 {
-	int ret;
 	struct internal_config *internal_conf =
 		eal_get_internal_configuration();
 	rte_cpuset_t *cpuset = &internal_conf->ctrl_cpuset;
@@ -184,11 +183,8 @@ static void *ctrl_thread_init(void *arg)
 
 	__rte_thread_init(rte_lcore_id(), cpuset);
 
-	ret = pthread_barrier_wait(&params->configured);
-	if (ret == PTHREAD_BARRIER_SERIAL_THREAD) {
-		pthread_barrier_destroy(&params->configured);
+	if (__atomic_sub_fetch(&params->refcnt, 1, __ATOMIC_ACQ_REL) == 0)
 		free(params);
-	}
 
 	return start_routine(routine_arg);
 }
@@ -210,14 +206,11 @@ rte_ctrl_thread_create(pthread_t *thread, const char *name,
 
 	params->start_routine = start_routine;
 	params->arg = arg;
-
-	pthread_barrier_init(&params->configured, NULL, 2);
+	params->refcnt = 2;
 
 	ret = pthread_create(thread, attr, ctrl_thread_init, (void *)params);
-	if (ret != 0) {
-		free(params);
-		return -ret;
-	}
+	if (ret != 0)
+		goto fail;
 
 	if (name != NULL) {
 		ret = rte_thread_setname(*thread, name);
@@ -228,24 +221,20 @@ rte_ctrl_thread_create(pthread_t *thread, const char *name,
 
 	ret = pthread_setaffinity_np(*thread, sizeof(*cpuset), cpuset);
 	if (ret)
-		goto fail;
+		goto fail_cancel;
 
-	ret = pthread_barrier_wait(&params->configured);
-	if (ret == PTHREAD_BARRIER_SERIAL_THREAD) {
-		pthread_barrier_destroy(&params->configured);
+	if (__atomic_sub_fetch(&params->refcnt, 1, __ATOMIC_ACQ_REL) == 0)
 		free(params);
-	}
 
 	return 0;
 
-fail:
-	if (PTHREAD_BARRIER_SERIAL_THREAD ==
-	    pthread_barrier_wait(&params->configured)) {
-		pthread_barrier_destroy(&params->configured);
-		free(params);
-	}
+fail_cancel:
 	pthread_cancel(*thread);
 	pthread_join(*thread, NULL);
+
+fail:
+	free(params);
+
 	return -ret;
 }
 
-- 
2.25.1