Bluetooth: hci_sync: don't drain cmd_sync backlog on unregister

Overview

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

Bluetooth: hci_sync: don't drain cmd_sync backlog on unregister

hci_unregister_dev() disables cmd_work and cmd_timer, then calls hci_cmd_sync_clear(), whose cancel_work_sync() waits for hci_cmd_sync_work() to return. That worker dequeues and runs every entry on cmd_sync_work_list. With cmd_work disabled nothing reaches the controller, so each queued HCI command waits the full HCI_CMD_TIMEOUT (2s).

The backlog has no bound. Userspace can keep queuing MGMT commands such as MGMT_OP_GET_CLOCK_INFO against a controller that doesn't answer. Closing /dev/vhci then blocks in vhci_release() for backlog * 2s:

INFO: task syz-executor:5749 blocked for more than 143 seconds. cancel_work_sync hci_cmd_sync_clear hci_unregister_dev vhci_release

In a local reproduction the backlog held more than 5000 entries, which comes to hours of hang.

Stop the worker from taking new entries once HCI_UNREGISTER is set, and wake any request still waiting with -ENODEV before cancelling the work. The entries left over are destroyed with -ECANCELED by the existing sweep in hci_cmd_sync_clear(). They stay on the list until the work has stopped, so hci_cmd_sync_dequeue() and friends still see them. A callback already running may still issue another command, which delays unregister by at most one timeout per remaining command, not by the whole backlog.

Affected

• Linux >= 008ee9eb8a11bcabf12c91771dd4f470b082bd44 < 143bde97c4101ae68eb656c87e620c45b07a3090
• Linux >= 008ee9eb8a11bcabf12c91771dd4f470b082bd44 < 7f16443a30f4e86d84ae4faf057d333b41ddb757
• Linux >= 008ee9eb8a11bcabf12c91771dd4f470b082bd44 < e8ba8c9c74d1035042cdf6cb5adf8f59b9b1271c
• Linux >= 008ee9eb8a11bcabf12c91771dd4f470b082bd44 < b1f24c1ab52345e810c42a1a81286cb5a7c92252
• Linux 5.17

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.