[PATCH RFC 2/2] USB: ldusb: fix ring-buffer locking

[Johan Hovold <johan@kernel.org>]

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

The lock did not cover the loads from the ring-buffer-entry after
determining the buffer was non-empty, nor the update of the tail index
once the entry had been processed. The former could lead to stale data
being returned, while the latter could lead to memory corruption on
sufficiently weakly ordered architectures.

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>
---
 drivers/usb/misc/ldusb.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/usb/misc/ldusb.c b/drivers/usb/misc/ldusb.c
index 94780e14e95d..c6cf2fa6cf4c 100644
--- a/drivers/usb/misc/ldusb.c
+++ b/drivers/usb/misc/ldusb.c
@@ -477,11 +477,11 @@ static ssize_t ld_usb_read(struct file *file, char __user *buffer, size_t count,
 
 		spin_lock_irq(&dev->rbsl);
 	}
-	spin_unlock_irq(&dev->rbsl);
 
 	/* actual_buffer contains actual_length + interrupt_in_buffer */
 	actual_buffer = (size_t *)(dev->ring_buffer + dev->ring_tail * (sizeof(size_t)+dev->interrupt_in_endpoint_size));
 	if (*actual_buffer > sizeof(size_t) + dev->interrupt_in_endpoint_size) {
+		spin_unlock_irq(&dev->rbsl);
 		retval = -EIO;
 		goto unlock_exit;
 	}
@@ -489,17 +489,26 @@ static ssize_t ld_usb_read(struct file *file, char __user *buffer, size_t count,
 	if (bytes_to_read < *actual_buffer)
 		dev_warn(&dev->intf->dev, "Read buffer overflow, %zd bytes dropped\n",
 			 *actual_buffer-bytes_to_read);
+	spin_unlock_irq(&dev->rbsl);
+
+	/*
+	 * Pairs with spin_unlock_irqrestore() in
+	 * ld_usb_interrupt_in_callback() and makes sure the ring-buffer entry
+	 * has been updated before copy_to_user().
+	 */
+	smp_rmb();
 
 	/* copy one interrupt_in_buffer from ring_buffer into userspace */
 	if (copy_to_user(buffer, actual_buffer+1, bytes_to_read)) {
 		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);
-- 
2.23.0