[PATCH 3/5] usb: xhci: Fix isochronous Ring Underrun/Overrun event handling

Michal Pecio
2025-02-10T07:40:44Z

The TRB pointer of these events points at enqueue at the time of error
occurrence on xHCI 1.1+ HCs or it's NULL on older ones. By the time we
are handling the event, a new TD may be queued at this ring position.

I can trigger this race by rising interrupt moderation to increase IRQ
handling delay. Similar delay may occur naturally due to system load.

If this ever happens after a Missed Service Error, missed TDs will be
skipped and the new TD processed as if it matched the event. It could
be given back prematurely, risking data loss or buffer UAF by the xHC.

Don't complete TDs on xrun events and don't warn if queued TDs don't
match the event's TRB pointer, which can be NULL or a link/no-op TRB.

Now that it's safe, also handle xrun events if the skip flag is clear.
This ensures completion of any TD stuck in 'error mid TD' state right
before the xrun event, which could happen if a driver submits a finite
number of URBs to a buggy HC and then an error occurs on the last TD.

Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
---
drivers/usb/host/xhci-ring.c | 14 ++++++++------
1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 9b06a911a16e..11a53e310826 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -2664,6 +2664,7 @@ static int handle_tx_event(struct xhci_hcd *xhci,
int status = -EINPROGRESS;
struct xhci_ep_ctx *ep_ctx;
u32 trb_comp_code;
+ bool ring_xrun_event = false;

slot_id = TRB_TO_SLOT_ID(le32_to_cpu(event->flags));
ep_index = TRB_TO_EP_ID(le32_to_cpu(event->flags)) - 1;
@@ -2770,14 +2771,12 @@ static int handle_tx_event(struct xhci_hcd *xhci,
* Underrun Event for OUT Isoch endpoint.
*/
xhci_dbg(xhci, "Underrun event on slot %u ep %u\n", slot_id, ep_index);
- if (ep->skip)
- break;
- return 0;
+ ring_xrun_event = true;
+ break;
case COMP_RING_OVERRUN:
xhci_dbg(xhci, "Overrun event on slot %u ep %u\n", slot_id, ep_index);
- if (ep->skip)
- break;
- return 0;
+ ring_xrun_event = true;
+ break;
case COMP_MISSED_SERVICE_ERROR:
/*
* When encounter missed service error, one or more isoc tds
@@ -2894,6 +2893,9 @@ static int handle_tx_event(struct xhci_hcd *xhci,
*/
} while (ep->skip);

+ if (ring_xrun_event)
+ return 0; /* don't warn or complete any TDs */
+
if (!ep_seg) {
/*
* Skip the Force Stopped Event. The 'ep_trb' of FSE is not in the current
--
2.48.1