usbip: fix vhci_hcd attach_store() races leading to gpf

attach_store() is invoked when user requests import (attach) a device
from usbip host.

Attach and detach are governed by local state and shared state
- Shared state (usbip device status) - Device status is used to manage
  the attach and detach operations on import-able devices.
- Local state (tcp_socket, rx and tx thread task_struct ptrs)
  A valid tcp_socket controls rx and tx thread operations while the
  device is in exported state.
- Device has to be in the right state to be attached and detached.

Attach sequence includes validating the socket and creating receive (rx)
and transmit (tx) threads to talk to the host to get access to the
imported device. rx and tx threads depends on local and shared state to
be correct and in sync.

Detach sequence shuts the socket down and stops the rx and tx threads.
Detach sequence relies on local and shared states to be in sync.

There are races in updating the local and shared status in the current
attach sequence resulting in crashes. These stem from starting rx and
tx threads before local and global state is updated correctly to be in
sync.

1. Doesn't handle kthread_create() error and saves invalid ptr in local
   state that drives rx and tx threads.
2. Updates tcp_socket and sockfd,  starts stub_rx and stub_tx threads
   before updating usbip_device status to VDEV_ST_NOTASSIGNED. This opens
   up a race condition between the threads, port connect, and detach
   handling.

Fix the above problems:
- Stop using kthread_get_run() macro to create/start threads.
- Create threads and get task struct reference.
- Add kthread_create() failure handling and bail out.
- Hold vhci and usbip_device locks to update local and shared states after
  creating rx and tx threads.
- Update usbip_device status to VDEV_ST_NOTASSIGNED.
- Update usbip_device tcp_socket, sockfd, tcp_rx, and tcp_tx
- Start threads after usbip_device (tcp_socket, sockfd, tcp_rx, tcp_tx,
  and status) is complete.

Credit goes to syzbot and Tetsuo Handa for finding and root-causing the
kthread_get_run() improper error handling problem and others. This is
hard problem to find and debug since the races aren't seen in a normal
case. Fuzzing forces the race window to be small enough for the
kthread_get_run() error path bug and starting threads before updating the
local and shared state bug in the attach sequence.
- Update usbip_device tcp_rx and tcp_tx pointers holding vhci and
  usbip_device locks.

Tested with syzbot reproducer:
- https://syzkaller.appspot.com/text?tag=ReproC&x=14801034d00000

Fixes: 9720b4b ("staging/usbip: convert to kthread")
Cc: stable@vger.kernel.org
Reported-by: syzbot <syzbot+a93fba6d384346a761e3@syzkaller.appspotmail.com>
Reported-by: syzbot <syzbot+bf1a360e305ee719e364@syzkaller.appspotmail.com>
Reported-by: syzbot <syzbot+95ce4b142579611ef0a9@syzkaller.appspotmail.com>
Reported-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/bb434bd5d7a64fbec38b5ecfb838a6baef6eb12b.1615171203.git.skhan@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: shuahkh

drivers/usb/usbip/vhci_sysfs.c

@@ -312,6 +312,8 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
 	struct vhci *vhci;
 	int err;
 	unsigned long flags;
+	struct task_struct *tcp_rx = NULL;
+	struct task_struct *tcp_tx = NULL;
 
 	/*
 	 * @rhport: port number of vhci_hcd
@@ -360,9 +362,24 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
 		return -EINVAL;
 	}
 
-	/* now need lock until setting vdev status as used */
+	/* create threads before locking */
+	tcp_rx = kthread_create(vhci_rx_loop, &vdev->ud, "vhci_rx");
+	if (IS_ERR(tcp_rx)) {
+		sockfd_put(socket);
+		return -EINVAL;
+	}
+	tcp_tx = kthread_create(vhci_tx_loop, &vdev->ud, "vhci_tx");
+	if (IS_ERR(tcp_tx)) {
+		kthread_stop(tcp_rx);
+		sockfd_put(socket);
+		return -EINVAL;
+	}
+
+	/* get task structs now */
+	get_task_struct(tcp_rx);
+	get_task_struct(tcp_tx);
 
-	/* begin a lock */
+	/* now begin lock until setting vdev status set */
 	spin_lock_irqsave(&vhci->lock, flags);
 	spin_lock(&vdev->ud.lock);
 
@@ -372,6 +389,8 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
 		spin_unlock_irqrestore(&vhci->lock, flags);
 
 		sockfd_put(socket);
+		kthread_stop_put(tcp_rx);
+		kthread_stop_put(tcp_tx);
 
 		dev_err(dev, "port %d already used\n", rhport);
 		/*
@@ -390,15 +409,17 @@ static ssize_t attach_store(struct device *dev, struct device_attribute *attr,
 	vdev->speed         = speed;
 	vdev->ud.sockfd     = sockfd;
 	vdev->ud.tcp_socket = socket;
+	vdev->ud.tcp_rx     = tcp_rx;
+	vdev->ud.tcp_tx     = tcp_tx;
 	vdev->ud.status     = VDEV_ST_NOTASSIGNED;
 	usbip_kcov_handle_init(&vdev->ud);
 
 	spin_unlock(&vdev->ud.lock);
 	spin_unlock_irqrestore(&vhci->lock, flags);
 	/* end the lock */
 
-	vdev->ud.tcp_rx = kthread_get_run(vhci_rx_loop, &vdev->ud, "vhci_rx");
-	vdev->ud.tcp_tx = kthread_get_run(vhci_tx_loop, &vdev->ud, "vhci_tx");
+	wake_up_process(vdev->ud.tcp_rx);
+	wake_up_process(vdev->ud.tcp_tx);
 
 	rh_port_connect(vdev, speed);