USB: usblp: fix a hang in poll() if disconnected

Apparently an application that opens a device and calls select()
on it, will hang if the decice is disconnected. It's a little
surprising that we had this bug for 15 years, but apparently
nobody ever uses select() with a printer: only write() and read(),
and those work fine. Well, you can also select() with a timeout.

The fix is modeled after devio.c. A few other drivers check the
condition first, then do not add the wait queue in case the
device is disconnected. We doubt that's completely race-free.
So, this patch adds the process first, then locks properly
and checks for the disconnect.

Reviewed-by: Zqiang <qiang.zhang@windriver.com>
Signed-off-by: Pete Zaitcev <zaitcev@redhat.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20210303221053.1cf3313e@suzdal.zaitcev.lan
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/class/usblp.c b/drivers/usb/class/usblp.c
index c9f6e97..f27b4ae 100644 (file)
--- a/drivers/usb/class/usblp.c
+++ b/drivers/usb/class/usblp.c
@@ -494,16 +494,24 @@ static int usblp_release(struct inode *inode, struct file *file)
 /* No kernel lock - fine */
 static __poll_t usblp_poll(struct file *file, struct poll_table_struct *wait)
 {
-       __poll_t ret;
+       struct usblp *usblp = file->private_data;
+       __poll_t ret = 0;
        unsigned long flags;
 
-       struct usblp *usblp = file->private_data;
        /* Should we check file->f_mode & FMODE_WRITE before poll_wait()? */
        poll_wait(file, &usblp->rwait, wait);
        poll_wait(file, &usblp->wwait, wait);
+
+       mutex_lock(&usblp->mut);
+       if (!usblp->present)
+               ret |= EPOLLHUP;
+       mutex_unlock(&usblp->mut);
+
        spin_lock_irqsave(&usblp->lock, flags);
-       ret = ((usblp->bidir && usblp->rcomplete) ? EPOLLIN  | EPOLLRDNORM : 0) |
-          ((usblp->no_paper || usblp->wcomplete) ? EPOLLOUT | EPOLLWRNORM : 0);
+       if (usblp->bidir && usblp->rcomplete)
+               ret |= EPOLLIN  | EPOLLRDNORM;
+       if (usblp->no_paper || usblp->wcomplete)
+               ret |= EPOLLOUT | EPOLLWRNORM;
        spin_unlock_irqrestore(&usblp->lock, flags);
        return ret;
 }