SUNRPC: Fix race corrupting rpc upcall

If rpc_queue_upcall() adds a new upcall to the rpci->pipe list just
after rpc_pipe_release calls rpc_purge_list(), but before it calls
gss_pipe_release (as rpci->ops->release_pipe(inode)), then the latter
will free a message without deleting it from the rpci->pipe list.

We will be left with a freed object on the rpc->pipe list.  Most
frequent symptoms are kernel crashes in rpc.gssd system calls on the
pipe in question.
Reported-by: NJ. Bruce Fields <bfields@redhat.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@kernel.org

net/sunrpc/auth_gss/auth_gss.c

@@ -745,17 +745,18 @@ gss_pipe_release(struct inode *inode)
 	struct rpc_inode *rpci = RPC_I(inode);
 	struct gss_upcall_msg *gss_msg;
 
+restart:
 	spin_lock(&inode->i_lock);
-	while (!list_empty(&rpci->in_downcall)) {
+	list_for_each_entry(gss_msg, &rpci->in_downcall, list) {
 
-		gss_msg = list_entry(rpci->in_downcall.next,
-				struct gss_upcall_msg, list);
+		if (!list_empty(&gss_msg->msg.list))
+			continue;
 		gss_msg->msg.errno = -EPIPE;
 		atomic_inc(&gss_msg->count);
 		__gss_unhash_msg(gss_msg);
 		spin_unlock(&inode->i_lock);
 		gss_release_msg(gss_msg);
-		spin_lock(&inode->i_lock);
+		goto restart;
 	}
 	spin_unlock(&inode->i_lock);
 

net/sunrpc/rpc_pipe.c

@@ -48,7 +48,7 @@ static void rpc_purge_list(struct rpc_inode *rpci, struct list_head *head,
 		return;
 	do {
 		msg = list_entry(head->next, struct rpc_pipe_msg, list);
-		list_del(&msg->list);
+		list_del_init(&msg->list);
 		msg->errno = err;
 		destroy_msg(msg);
 	} while (!list_empty(head));
@@ -208,7 +208,7 @@ rpc_pipe_release(struct inode *inode, struct file *filp)
 	if (msg != NULL) {
 		spin_lock(&inode->i_lock);
 		msg->errno = -EAGAIN;
-		list_del(&msg->list);
+		list_del_init(&msg->list);
 		spin_unlock(&inode->i_lock);
 		rpci->ops->destroy_msg(msg);
 	}
@@ -268,7 +268,7 @@ rpc_pipe_read(struct file *filp, char __user *buf, size_t len, loff_t *offset)
 	if (res < 0 || msg->len == msg->copied) {
 		filp->private_data = NULL;
 		spin_lock(&inode->i_lock);
-		list_del(&msg->list);
+		list_del_init(&msg->list);
 		spin_unlock(&inode->i_lock);
 		rpci->ops->destroy_msg(msg);
 	}