xhci: Make handover code more robust

My test platform (Intel DX79SI) boots reliably under BIOS, but frequently
crashes when booting via UEFI. I finally tracked this down to the xhci
handoff code. It seems that reads from the device occasionally just return
0xff, resulting in xhci_find_next_cap_offset generating a value that's
larger than the resource region. We then oops when attempting to read the
value. Sanity checking that value lets us avoid the crash.

I've no idea what's causing the underlying problem, and xhci still doesn't
actually *work* even with this, but the machine at least boots which will
probably make further debugging easier.

This should be backported to kernels as old as 2.6.31, that contain the
commit 66d4ead "USB: xhci: BIOS handoff
and HW initialization."

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org

drivers/usb/host/pci-quirks.c

@@ -841,12 +841,12 @@ static void __devinit quirk_usb_handoff_xhci(struct pci_dev *pdev)
 	void __iomem *op_reg_base;
 	u32 val;
 	int timeout;
+	int len = pci_resource_len(pdev, 0);
 
 	if (!mmio_resource_enabled(pdev, 0))
 		return;
 
-	base = ioremap_nocache(pci_resource_start(pdev, 0),
-				pci_resource_len(pdev, 0));
+	base = ioremap_nocache(pci_resource_start(pdev, 0), len);
 	if (base == NULL)
 		return;
 
@@ -856,9 +856,17 @@ static void __devinit quirk_usb_handoff_xhci(struct pci_dev *pdev)
 	 */
 	ext_cap_offset = xhci_find_next_cap_offset(base, XHCI_HCC_PARAMS_OFFSET);
 	do {
+		if ((ext_cap_offset + sizeof(val)) > len) {
+			/* We're reading garbage from the controller */
+			dev_warn(&pdev->dev,
+				 "xHCI controller failing to respond");
+			return;
+		}
+
 		if (!ext_cap_offset)
 			/* We've reached the end of the extended capabilities */
 			goto hc_init;
+
 		val = readl(base + ext_cap_offset);
 		if (XHCI_EXT_CAPS_ID(val) == XHCI_EXT_CAPS_LEGACY)
 			break;