| Message ID | 20250226080255.770ca055@foxbook (mailing list archive) |
|---|---|
| State | Accepted |
| Commit | 58d0a3fab5f4fdc112c16a4c6d382f62097afd1c |
| Headers | show |
| Series | xHCI: Isochronous error handling fixes and improvements | expand |
On Wed, 26 Feb 2025 08:02:55 +0100, Michal Pecio wrote: > After d56b0b2ab142, TDs are immediately skipped when handling those > Stopped events. This poses a potential problem in case of Stopped - > Length Invalid, which occurs either on completed TDs (likely already > given back) or Link and No-Op TRBs. Such event won't be recognized > as matching any TD (unless it's the rare Link TRB inside a TD) and > will result in skipping all pending TDs, giving them back possibly > before they are done, risking isoc data loss and maybe UAF by HW. Actually, Stopped and Stopped - Short Packet may be unsafe too. As far as I understand, one of those (depending on SPC capability) can occur on the second TRB of a TD whose first TRB completed with Short Packet. Then the TD is already given back and removed from td_list, so no match will be found with this Stopped event. I suspect this is the reason why the driver has a policy to silently ignore Stopped events which don't match the pending TD, and not only Stopped - Length Invalid. Not sure why Stopped - Short Packet isn't also ignored and yet apparently doesn't cause problems.
diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c index 67f3d8128b10..96b90819aec7 100644 --- a/drivers/usb/host/xhci-ring.c +++ b/drivers/usb/host/xhci-ring.c @@ -2870,6 +2870,10 @@ static int handle_tx_event(struct xhci_hcd *xhci, if (!ep_seg) { if (ep->skip && usb_endpoint_xfer_isoc(&td->urb->ep->desc)) { + /* this event is unlikely to match any TD, don't skip them all */ + if (trb_comp_code == COMP_STOPPED_LENGTH_INVALID) + return 0; + skip_isoc_td(xhci, td, ep, status); if (!list_empty(&ep_ring->td_list)) continue;
Up until commit d56b0b2ab142 ("usb: xhci: ensure skipped isoc TDs are returned when isoc ring is stopped") in v6.11, the driver didn't skip missed isochronous TDs when handling Stoppend and Stopped - Length Invalid events. Instead, it erroneously cleared the skip flag, which would cause the ring to get stuck, as future events won't match the missed TD which is never removed from the queue until it's cancelled. This buggy logic seems to have been in place substantially unchanged since the 3.x series over 10 years ago, which probably speaks first and foremost about relative rarity of this case in normal usage, but by the spec I see no reason why it shouldn't be possible. After d56b0b2ab142, TDs are immediately skipped when handling those Stopped events. This poses a potential problem in case of Stopped - Length Invalid, which occurs either on completed TDs (likely already given back) or Link and No-Op TRBs. Such event won't be recognized as matching any TD (unless it's the rare Link TRB inside a TD) and will result in skipping all pending TDs, giving them back possibly before they are done, risking isoc data loss and maybe UAF by HW. As a compromise, don't skip and don't clear the skip flag on this kind of event. Then the next event will skip missed TDs. A downside of not handling Stopped - Length Invalid on a Link inside a TD is that if the TD is cancelled, its actual length will not be updated to account for TRBs (silently) completed before the TD was stopped. I had no luck producing this sequence of completion events so there is no compelling demonstration of any resulting disaster. It may be a very rare, obscure condition. The sole motivation for this patch is that if such unlikely event does occur, I'd rather risk reporting a cancelled partially done isoc frame as empty than gamble with UAF. This will be fixed more properly by looking at Stopped event's TRB pointer when making skipping decisions, but such rework is unlikely to be backported to v6.12, which will stay around for a few years. Fixes: d56b0b2ab142 ("usb: xhci: ensure skipped isoc TDs are returned when isoc ring is stopped") Cc: stable@vger.kernel.org Signed-off-by: Michal Pecio <michal.pecio@gmail.com> --- drivers/usb/host/xhci-ring.c | 4 ++++ 1 file changed, 4 insertions(+)