SUNRPC: Track writers of the 'channel' file to improve cache_listeners_exist

The sunrpc cache interface is susceptible to being fooled by a rogue
process just reading a 'channel' file.  If this happens the kernel
may think a valid daemon exists to service the cache when it does not.
For example, the following may fool the kernel:
cat /proc/net/rpc/auth.unix.gid/channel

Change the tracking of readers to writers when considering whether a
listener exists as all valid daemon processes either open a channel
file O_RDWR or O_WRONLY.  While this does not prevent a rogue process
from "stealing" a message from the kernel, it does at least improve
the kernels perception of whether a valid process servicing the cache
exists.
Signed-off-by: Dave Wysochanski <dwysocha@redhat.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

include/linux/sunrpc/cache.h

@@ -107,9 +107,9 @@ struct cache_detail {
 	/* fields for communication over channel */
 	struct list_head	queue;
 
-	atomic_t		readers;		/* how many time is /chennel open */
-	time_t			last_close;		/* if no readers, when did last close */
-	time_t			last_warn;		/* when we last warned about no readers */
+	atomic_t		writers;		/* how many time is /channel open */
+	time_t			last_close;		/* if no writers, when did last close */
+	time_t			last_warn;		/* when we last warned about no writers */
 
 	union {
 		struct proc_dir_entry	*procfs;

net/sunrpc/cache.c

@@ -373,7 +373,7 @@ void sunrpc_init_cache_detail(struct cache_detail *cd)
 	spin_lock(&cache_list_lock);
 	cd->nextcheck = 0;
 	cd->entries = 0;
-	atomic_set(&cd->readers, 0);
+	atomic_set(&cd->writers, 0);
 	cd->last_close = 0;
 	cd->last_warn = -1;
 	list_add(&cd->others, &cache_list);
@@ -1029,11 +1029,13 @@ static int cache_open(struct inode *inode, struct file *filp,
 		}
 		rp->offset = 0;
 		rp->q.reader = 1;
-		atomic_inc(&cd->readers);
+
 		spin_lock(&queue_lock);
 		list_add(&rp->q.list, &cd->queue);
 		spin_unlock(&queue_lock);
 	}
+	if (filp->f_mode & FMODE_WRITE)
+		atomic_inc(&cd->writers);
 	filp->private_data = rp;
 	return 0;
 }
@@ -1062,8 +1064,10 @@ static int cache_release(struct inode *inode, struct file *filp,
 		filp->private_data = NULL;
 		kfree(rp);
 
+	}
+	if (filp->f_mode & FMODE_WRITE) {
+		atomic_dec(&cd->writers);
 		cd->last_close = seconds_since_boot();
-		atomic_dec(&cd->readers);
 	}
 	module_put(cd->owner);
 	return 0;
@@ -1171,7 +1175,7 @@ static void warn_no_listener(struct cache_detail *detail)
 
 static bool cache_listeners_exist(struct cache_detail *detail)
 {
-	if (atomic_read(&detail->readers))
+	if (atomic_read(&detail->writers))
 		return true;
 	if (detail->last_close == 0)
 		/* This cache was never opened */