CVE-2024-40927

In the Linux kernel, the following vulnerability has been resolved:

xhci: Handle TD clearing for multiple streams case

When multiple streams are in use, multiple TDs might be in flight when an endpoint is stopped. We need to issue a Set TR Dequeue Pointer for each, to ensure everything is reset properly and the caches cleared. Change the logic so that any N>1 TDs found active for different streams are deferred until after the first one is processed, calling xhci_invalidate_cancelled_tds() again from xhci_handle_cmd_set_deq() to queue another command until we are done with all of them. Also change the error/should never happen paths to ensure we at least clear any affected TDs, even if we cant issue a command to clear the hardware cache, and complain loudly with an xhci_warn() if this ever happens.

This problem case dates back to commit e9df17eb1408 (USB: xhci: Correct assumptions about number of rings per endpoint.) early on in the XHCI drivers life, when stream support was first added. It was then identified but not fixed nor made into a warning in commit 674f8438c121 (xhci: split handling halted endpoints into two steps), which added a FIXME comment for the problem case (without materially changing the behavior as far as I can tell, though the new logic made the problem more obvious).

Then later, in commit 94f339147fc3 (xhci: Fix failure to give back some cached cancelled URBs.), it was acknowledged again.

[Mathias: commit 94f339147fc3 (xhci: Fix failure to give back some cached cancelled URBs.) was a targeted regression fix to the previously mentioned patch. Users reported issues with usb stuck after unmounting/disconnecting UAS devices. This rolled back the TD clearing of multiple streams to its original state.]

Apparently the commit author was aware of the problem (yet still chose to submit it): It was still mentioned as a FIXME, an xhci_dbg() was added to log the problem condition, and the remaining issue was mentioned in the commit description. The choice of making the log type xhci_dbg() for what is, at this point, a completely unhandled and known broken condition is puzzling and unfortunate, as it guarantees that no actual users would see the log in production, thereby making it nigh undebuggable (indeed, even if you turn on DEBUG, the message doesnt really hint at there being a problem at all).

It took me months of random xHC crashes to finally find a reliable repro and be able to do a deep dive debug session, which could all have been avoided had this unhandled, broken condition been actually reported with a warning, as it should have been as a bug intentionally left in unfixed (never mind that it shouldnt have been left in at all).

Another fix to solve clearing the caches of all stream rings with cancelled TDs is needed, but not as urgent.

3 years after that statement and 14 years after the original bug was introduced, I think its finally time to fix it. And maybe next time lets not leave bugs unfixed (that are actually worse than the original bug), and lets actually get people to review kernel commits please.

Fixes xHC crashes and IOMMU faults with UAS devices when handling errors/faults. Easiest repro is to use hdparm to mark an early sector (e.g. 1024) on a disk as bad, then cat /dev/sdX > /dev/null in a loop. At least in the case of JMicron controllers, the read errors end up having to cancel two TDs (for two queued requests to different streams) and the one that didnt get cleared properly ends up faulting the xHC entirely when it tries to access DMA pages that have since been unmapped, referred to by the stale TDs. This normally happens quickly (after two or three loops). After this fix, I left the cat in a loop running overnight and experienced no xHC failures, with all read errors recovered properly. Reprod and tested on an Apple M1 Mac Mini (dwc3 host).

On systems without an IOMMU, this bug would instead silently corrupt freed memory, making this a —truncated—

Affected Software

References

• https://git.kernel.org/stable/c/26460c1afa311524f588e288a4941432f0de6228
• https://git.kernel.org/stable/c/5ceac4402f5d975e5a01c806438eb4e554771577
• https://git.kernel.org/stable/c/61593dc413c3655e4328a351555235bc3089486a
• https://git.kernel.org/stable/c/633f72cb6124ecda97b641fbc119340bd88d51a9
• https://git.kernel.org/stable/c/949be4ec5835e0ccb3e2a8ab0e46179cb5512518