[PATCH 2/2] xhci: Fix use after free for URB cancellation on a reallocated endpoint

[Mathias Nyman <mathias.nyman@linux.intel.com>]

Make sure the cancelled URB is on the current endpoint ring.

If the endpoint ring has been reallocated since the URB was enqueued
then the URB may contain TD and TRB pointers to a already freed ring.
In this the case return the URB without touching any of the freed ring
structure data.

Don't try to stop the ring. It would be useless.

This can occur if endpoint is not flushed before it is dropped and
re-added, which is the case in usb_set_interface() as xhci does
things in an odd order.

Cc: <stable@vger.kernel.org>
Tested-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
---
 drivers/usb/host/xhci.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/drivers/usb/host/xhci.c b/drivers/usb/host/xhci.c
index 61f48b1..0420eef 100644
--- a/drivers/usb/host/xhci.c
+++ b/drivers/usb/host/xhci.c
@@ -37,6 +37,21 @@ static unsigned long long quirks;
 module_param(quirks, ullong, S_IRUGO);
 MODULE_PARM_DESC(quirks, "Bit flags for quirks to be enabled as default");
 
+static bool td_on_ring(struct xhci_td *td, struct xhci_ring *ring)
+{
+	struct xhci_segment *seg = ring->first_seg;
+
+	if (!td || !td->start_seg)
+		return false;
+	do {
+		if (seg == td->start_seg)
+			return true;
+		seg = seg->next;
+	} while (seg && seg != ring->first_seg);
+
+	return false;
+}
+
 /* TODO: copied from ehci-hcd.c - can this be refactored? */
 /*
  * xhci_handshake - spin reading hc until handshake completes or fails
@@ -1571,6 +1586,21 @@ static int xhci_urb_dequeue(struct usb_hcd *hcd, struct urb *urb, int status)
 		goto done;
 	}
 
+	/*
+	 * check ring is not re-allocated since URB was enqueued. If it is, then
+	 * make sure none of the ring related pointers in this URB private data
+	 * are touched, such as td_list, otherwise we overwrite freed data
+	 */
+	if (!td_on_ring(&urb_priv->td[0], ep_ring)) {
+		xhci_err(xhci, "Canceled URB td not found on endpoint ring");
+		for (i = urb_priv->num_tds_done; i < urb_priv->num_tds; i++) {
+			td = &urb_priv->td[i];
+			if (!list_empty(&td->cancelled_td_list))
+				list_del_init(&td->cancelled_td_list);
+		}
+		goto err_giveback;
+	}
+
 	if (xhci->xhc_state & XHCI_STATE_HALTED) {
 		xhci_dbg_trace(xhci, trace_xhci_dbg_cancel_urb,
 				"HC halted, freeing TD manually.");
-- 
2.7.4