cachefiles: add spin_lock for cachefiles_ondemand_info

The following concurrency may cause a read request to fail to be completed
and result in a hung:

           t1             |             t2
---------------------------------------------------------
                            cachefiles_ondemand_copen
                              req = xa_erase(&cache->reqs, id)
// Anon fd is maliciously closed.
cachefiles_ondemand_fd_release
  xa_lock(&cache->reqs)
  cachefiles_ondemand_set_object_close(object)
  xa_unlock(&cache->reqs)
                              cachefiles_ondemand_set_object_open
                              // No one will ever close it again.
cachefiles_ondemand_daemon_read
  cachefiles_ondemand_select_req
  // Get a read req but its fd is already closed.
  // The daemon can't issue a cread ioctl with an closed fd, then hung.

So add spin_lock for cachefiles_ondemand_info to protect ondemand_id and
state, thus we can avoid the above problem in cachefiles_ondemand_copen()
by using ondemand_id to determine if fd has been closed.

Fixes: c8383054 ("cachefiles: notify the user daemon when looking up cookie")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240522114308.2402121-8-libaokun@huaweicloud.comAcked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/cachefiles/internal.h

@@ -55,6 +55,7 @@ struct cachefiles_ondemand_info {
 	int				ondemand_id;
 	enum cachefiles_object_state	state;
 	struct cachefiles_object	*object;
+	spinlock_t			lock;
 };
 
 /*

fs/cachefiles/ondemand.c

@@ -16,13 +16,16 @@ static int cachefiles_ondemand_fd_release(struct inode *inode,
 	struct cachefiles_object *object = file->private_data;
 	struct cachefiles_cache *cache = object->volume->cache;
 	struct cachefiles_ondemand_info *info = object->ondemand;
-	int object_id = info->ondemand_id;
+	int object_id;
 	struct cachefiles_req *req;
 	XA_STATE(xas, &cache->reqs, 0);
 
 	xa_lock(&cache->reqs);
+	spin_lock(&info->lock);
+	object_id = info->ondemand_id;
 	info->ondemand_id = CACHEFILES_ONDEMAND_ID_CLOSED;
 	cachefiles_ondemand_set_object_close(object);
+	spin_unlock(&info->lock);
 
 	/* Only flush CACHEFILES_REQ_NEW marked req to avoid race with daemon_read */
 	xas_for_each_marked(&xas, req, ULONG_MAX, CACHEFILES_REQ_NEW) {
@@ -127,6 +130,7 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
 {
 	struct cachefiles_req *req;
 	struct fscache_cookie *cookie;
+	struct cachefiles_ondemand_info *info;
 	char *pid, *psize;
 	unsigned long id;
 	long size;
@@ -185,6 +189,33 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
 		goto out;
 	}
 
+	info = req->object->ondemand;
+	spin_lock(&info->lock);
+	/*
+	 * The anonymous fd was closed before copen ? Fail the request.
+	 *
+	 *             t1             |             t2
+	 * ---------------------------------------------------------
+	 *                             cachefiles_ondemand_copen
+	 *                             req = xa_erase(&cache->reqs, id)
+	 * // Anon fd is maliciously closed.
+	 * cachefiles_ondemand_fd_release
+	 * xa_lock(&cache->reqs)
+	 * cachefiles_ondemand_set_object_close(object)
+	 * xa_unlock(&cache->reqs)
+	 *                             cachefiles_ondemand_set_object_open
+	 *                             // No one will ever close it again.
+	 * cachefiles_ondemand_daemon_read
+	 * cachefiles_ondemand_select_req
+	 *
+	 * Get a read req but its fd is already closed. The daemon can't
+	 * issue a cread ioctl with an closed fd, then hung.
+	 */
+	if (info->ondemand_id == CACHEFILES_ONDEMAND_ID_CLOSED) {
+		spin_unlock(&info->lock);
+		req->error = -EBADFD;
+		goto out;
+	}
 	cookie = req->object->cookie;
 	cookie->object_size = size;
 	if (size)
@@ -194,6 +225,7 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
 	trace_cachefiles_ondemand_copen(req->object, id, size);
 
 	cachefiles_ondemand_set_object_open(req->object);
+	spin_unlock(&info->lock);
 	wake_up_all(&cache->daemon_pollwq);
 
 out:
@@ -596,6 +628,7 @@ int cachefiles_ondemand_init_obj_info(struct cachefiles_object *object,
 		return -ENOMEM;
 
 	object->ondemand->object = object;
+	spin_lock_init(&object->ondemand->lock);
 	INIT_WORK(&object->ondemand->ondemand_work, ondemand_object_worker);
 	return 0;
 }