[dm-devel] [PATCH v2 29/29] libmultipath: fix race between log_safe and log_thread_stop()

[mwilck at suse.com]

From: Martin Wilck <mwilck at suse.com>

log_safe() could race with log_thread_stop(); simply
checking the value of log_thr has never been safe. By converting the
mutexes to static initializers, we avoid having to destroy them, and thus
possibly accessing a destroyed mutex in log_safe(). Furthermore, taking
both the logev_lock and the logq_lock makes sure the logarea isn't freed
while we are writing to it.

Signed-off-by: Martin Wilck <mwilck at suse.com>
---
 libmultipath/log_pthread.c | 39 ++++++++++++++++++++++++--------------
 1 file changed, 25 insertions(+), 14 deletions(-)

diff --git a/libmultipath/log_pthread.c b/libmultipath/log_pthread.c
index 3c73941..91c9c19 100644
--- a/libmultipath/log_pthread.c
+++ b/libmultipath/log_pthread.c
@@ -17,31 +17,42 @@
 
 static pthread_t log_thr;
 
-static pthread_mutex_t logq_lock;
-static pthread_mutex_t logev_lock;
-static pthread_cond_t logev_cond;
+/* logev_lock must not be taken with logq_lock held */
+static pthread_mutex_t logq_lock = PTHREAD_MUTEX_INITIALIZER;
+static pthread_mutex_t logev_lock = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t logev_cond = PTHREAD_COND_INITIALIZER;
 
 static int logq_running;
 static int log_messages_pending;
 
 void log_safe (int prio, const char * fmt, va_list ap)
 {
+	bool running;
+
 	if (prio > LOG_DEBUG)
 		prio = LOG_DEBUG;
 
-	if (log_thr == (pthread_t)0) {
-		vsyslog(prio, fmt, ap);
-		return;
-	}
+	/*
+	 * logev_lock protects logq_running. By holding it, we avoid a race
+	 * with log_thread_stop() -> log_close(), which would free the logarea.
+	 */
+	pthread_mutex_lock(&logev_lock);
+	pthread_cleanup_push(cleanup_mutex, &logev_lock);
+	running = logq_running;
 
-	pthread_mutex_lock(&logq_lock);
-	log_enqueue(prio, fmt, ap);
-	pthread_mutex_unlock(&logq_lock);
+	if (running) {
+		pthread_mutex_lock(&logq_lock);
+		pthread_cleanup_push(cleanup_mutex, &logq_lock);
+		log_enqueue(prio, fmt, ap);
+		pthread_cleanup_pop(1);
 
-	pthread_mutex_lock(&logev_lock);
-	log_messages_pending = 1;
-	pthread_cond_signal(&logev_cond);
-	pthread_mutex_unlock(&logev_lock);
+		log_messages_pending = 1;
+		pthread_cond_signal(&logev_cond);
+	}
+	pthread_cleanup_pop(1);
+
+	if (!running)
+		vsyslog(prio, fmt, ap);
 }
 
 static void flush_logqueue (void)
-- 
2.28.0