On Thu, 7 Dec 2017, Aaron Armstrong Skomra wrote:
Background: ExpressKey Remotes communicate their events via usb dongle. Each dongle can hold up to 5 pairings at one time and one EKR (identified by its serial number) can unfortunately be paired with its dongle more than once. The pairing takes place in a round-robin fashion.
Input devices are only created once per EKR, when a new serial number is seen in the list of pairings. However, if a device is created for a "higher" paring index and subsequently a second pairing occurs at a lower pairing index, unpairing the remote with that serial number from any pairing index will currently cause a driver crash. This occurs infrequently, as two remotes are necessary to trigger this bug and most users have only one remote.
As an illustration, to trigger the bug you need to have two remotes, and pair them in this order:
slot 0 -> remote 1 (input device created for remote 1)
slot 1 -> remote 1 (duplicate pairing - no device created)
slot 2 -> remote 1 (duplicate pairing - no device created)
slot 3 -> remote 1 (duplicate pairing - no device created)
slot 4 -> remote 2 (input device created for remote 2)
slot 0 -> remote 2 (1 destroyed and recreated at slot 1)
slot 1 -> remote 2 (1 destroyed and recreated at slot 2)
slot 2 -> remote 2 (1 destroyed and recreated at slot 3)
slot 3 -> remote 2 (1 destroyed and not recreated)
slot 4 -> remote 2 (2 was already in this slot so no changes)
slot 0 -> remote 1 (The current code sees remote 2 was paired over in one of the dongle slots it occupied and attempts to remove all information about remote 2 [1]. It calls wacom_remote_destroy_one for remote 2, but the destroy function assumes the lowest index is where the remote's input device was created. The code "cleans up" the other remote 2 pairings including the one which the input device was based on, assuming they were were just duplicate pairings. However, the cleanup doesn't call the devres release function for the input device that was created in slot 4).
This issue is fixed by this commit.
[1] Remote 2 should subsequently be re-created on the next packet from the EKR at the lowest numbered slot that it occupies (here slot 1).
Fixes: f9036bd43602 ("HID: wacom: EKR: use devres groups to manage resources") Cc: stable stable@vger.kernel.org #4.9 Signed-off-by: Aaron Armstrong Skomra aaron.skomra@wacom.com
Applied, thanks.