usb: xhci: Fix invalid pointer dereference in Etron workaround

This check is performed before prepare_transfer() and prepare_ring(), so
enqueue can already point at the final link TRB of a segment. And indeed
it will, some 0.4% of times this code is called.

Then enqueue + 1 is an invalid pointer. It will crash the kernel right
away or load some junk which may look like a link TRB and cause the real
link TRB to be replaced with a NOOP. This wouldn't end well.

Use a functionally equivalent test which doesn't dereference the pointer
and always gives correct result.

Something has crashed my machine twice in recent days while playing with
an Etron HC, and a control transfer stress test ran for confirmation has
just crashed it again. The same test passes with this patch applied.

Fixes: 5e1c67abc930 ("xhci: Fix control transfer error on Etron xHCI host")
Cc: stable@vger.kernel.org
Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Reviewed-by: Kuangyi Chiang <ki.chiang65@gmail.com>
Link: https://lore.kernel.org/r/20250410151828.2868740-5-mathias.nyman@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 4e975caca2359e696b78885a5c29c41099e2c55b..b906bc2eea5f6f52ab4cd4af171099de0a299f62 100644 (file)
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -3777,7 +3777,7 @@ int xhci_queue_ctrl_tx(struct xhci_hcd *xhci, gfp_t mem_flags,
                 * enqueue a No Op TRB, this can prevent the Setup and Data Stage
                 * TRB to be breaked by the Link TRB.
                 */
-               if (trb_is_link(ep_ring->enqueue + 1)) {
+               if (last_trb_on_seg(ep_ring->enq_seg, ep_ring->enqueue + 1)) {
                        field = TRB_TYPE(TRB_TR_NOOP) | ep_ring->cycle_state;
                        queue_trb(xhci, ep_ring, false, 0, 0,
                                        TRB_INTR_TARGET(0), field);