USB: ldusb: fix ring-buffer locking

commit d98ee2a19c3334e9343df3ce254b496f1fc428eb upstream.

The custom ring-buffer implementation was merged without any locking or
explicit memory barriers, but a spinlock was later added by commit
9d33efd9a791 ("USB: ldusb bugfix").

The lock did not cover the update of the tail index once the entry had
been processed, something which could lead to memory corruption on
weakly ordered architectures or due to compiler optimisations.

Specifically, a completion handler running on another CPU might observe
the incremented tail index and update the entry before ld_usb_read() is
done with it.

Fixes: 2824bd250f0b ("[PATCH] USB: add ldusb driver")
Fixes: 9d33efd9a791 ("USB: ldusb bugfix")
Cc: stable <stable@vger.kernel.org>     # 2.6.13
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191022143203.5260-2-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/misc/ldusb.c b/drivers/usb/misc/ldusb.c
index 52e28b4913ad8e0f0725cef6478b9dee52d3c2e9..e04795ee17c8cfe25088b606c0e88544d6d262f5 100644 (file)
--- a/drivers/usb/misc/ldusb.c
+++ b/drivers/usb/misc/ldusb.c
@@ -499,11 +499,11 @@ static ssize_t ld_usb_read(struct file *file, char __user *buffer, size_t count,
                retval = -EFAULT;
                goto unlock_exit;
        }
-       dev->ring_tail = (dev->ring_tail+1) % ring_buffer_size;
-
        retval = bytes_to_read;
 
        spin_lock_irq(&dev->rbsl);
+       dev->ring_tail = (dev->ring_tail + 1) % ring_buffer_size;
+
        if (dev->buffer_overflow) {
                dev->buffer_overflow = 0;
                spin_unlock_irq(&dev->rbsl);