cdc-wdm: Fix more races on the read path

We must not allow the input buffer length to change while we're
shuffling the buffer contents.  We also mustn't clear the WDM_READ
flag after more data might have arrived.  Therefore move both of these
into the spinlocked region at the bottom of wdm_read().

When reading desc->length without holding the iuspin lock, use
ACCESS_ONCE() to ensure the compiler doesn't re-read it with
inconsistent results.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Bjørn Mork <bjorn@mork.no>
Cc: Oliver Neukum <oliver@neukum.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 11 insertions, 5 deletions

diff --git a/drivers/usb/class/cdc-wdm.c b/drivers/usb/class/cdc-wdm.c
index b27bbb957e16..7ca54d4dea92 100644
--- a/drivers/usb/class/cdc-wdm.c
+++ b/drivers/usb/class/cdc-wdm.c
@@ -391,7 +391,7 @@ outnl:
 static ssize_t wdm_read
 (struct file *file, char __user *buffer, size_t count, loff_t *ppos)
 {
-        int rv, cntr = 0;
+        int rv, cntr;
         int i = 0;
         struct wdm_device *desc = file->private_data;
 
@@ -400,7 +400,8 @@ static ssize_t wdm_read
         if (rv < 0)
                 return -ERESTARTSYS;
 
-        if (desc->length == 0) {
+        cntr = ACCESS_ONCE(desc->length);
+        if (cntr == 0) {
                 desc->read = 0;
 retry:
                 if (test_bit(WDM_DISCONNECTING, &desc->flags)) {
@@ -455,25 +456,30 @@ retry:
                         goto retry;
                 }
                 clear_bit(WDM_READ, &desc->flags);
+                cntr = desc->length;
                 spin_unlock_irq(&desc->iuspin);
         }
 
-        cntr = count > desc->length ? desc->length : count;
+        if (cntr > count)
+                cntr = count;
         rv = copy_to_user(buffer, desc->ubuf, cntr);
         if (rv > 0) {
                 rv = -EFAULT;
                 goto err;
         }
 
+        spin_lock_irq(&desc->iuspin);
+
         for (i = 0; i < desc->length - cntr; i++)
                 desc->ubuf[i] = desc->ubuf[i + cntr];
 
-        spin_lock_irq(&desc->iuspin);
         desc->length -= cntr;
-        spin_unlock_irq(&desc->iuspin);
         /* in case we had outstanding data */
         if (!desc->length)
                 clear_bit(WDM_READ, &desc->flags);
+
+        spin_unlock_irq(&desc->iuspin);
+
         rv = cntr;
 
 err: