Re: [dm-devel] [PATCH v3 29/29] libmultipath: fix race between log_safe and log_thread_stop()

[Benjamin Marzinski <bmarzins@redhat.com>]

On Wed, Dec 16, 2020 at 07:17:08PM +0100, mwilck@suse.com wrote:
> From: Martin Wilck <mwilck@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.
> 
Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>
> Signed-off-by: Martin Wilck <mwilck@suse.com>
> ---
>  libmultipath/log_pthread.c | 48 +++++++++++++++++++++-----------------
>  1 file changed, 26 insertions(+), 22 deletions(-)
> 
> diff --git a/libmultipath/log_pthread.c b/libmultipath/log_pthread.c
> index 3a2566a..0d48c52 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)
> @@ -103,9 +114,6 @@ void log_thread_start (pthread_attr_t *attr)
>  	int running = 0;
>  
>  	logdbg(stderr,"enter log_thread_start\n");
> -	pthread_mutex_init(&logq_lock, NULL);
> -	pthread_mutex_init(&logev_lock, NULL);
> -	pthread_cond_init(&logev_cond, NULL);
>  
>  	if (log_init("multipathd", 0)) {
>  		fprintf(stderr,"can't initialize log buffer\n");
> @@ -154,13 +162,9 @@ void log_thread_stop (void)
>  	}
>  	pthread_cleanup_pop(1);
>  
> -	flush_logqueue();
>  	if (running)
>  		pthread_join(log_thr, NULL);
>  
> -	pthread_mutex_destroy(&logq_lock);
> -	pthread_mutex_destroy(&logev_lock);
> -	pthread_cond_destroy(&logev_cond);
> -
> +	flush_logqueue();
>  	log_close();
>  }
> -- 
> 2.29.0

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