In the Linux kernel, the following vulnerability has been resolved: Bluetooth: hci_uart: fix UAFs and race conditions in close and init paths Vulnerabilities leading to Use-After-Free (UAF) and Null Pointer Dereference (NPD) conditions…

Overview

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

Bluetooth: hci_uart: fix UAFs and race conditions in close and init paths

Vulnerabilities leading to Use-After-Free (UAF) and Null Pointer Dereference (NPD) conditions were observed in the lifecycle management of hci_uart.

The primary issue arises because the workqueues (init_ready and write_work) are only flushed/cancelled if the HCI_UART_PROTO_READY flag is set during TTY close. If a hangup occurs before setup completes, hci_uart_tty_close() skips the teardown of these workqueues and proceeds to free the hu struct. When the scheduled work executes later, it blindly dereferences the freed hu struct.

Furthermore, several data races and UAFs were identified in the teardown sequence:

1. Calling hci_uart_flush() from hci_uart_close() without effectively disabling write_work causes a race condition where both can concurrently double-free hu->tx_skb. This happens because protocol timers can concurrently invoke hci_uart_tx_wakeup() and requeue write_work.
2. Calling hci_free_dev(hdev) before hu->proto->close(hu) causes a UAF when vendor specific protocol close callbacks dereference hu->hdev.
3. In the initialization error paths, failing to take the proto_lock write lock before clearing PROTO_READY leads to races with active readers. Additionally, hci_uart_tty_receive() accesses hu->hdev outside the read lock, leading to UAFs if the initialization error path frees hdev concurrently.

Fix these synchronization and lifecycle issues by:

1. Re-ordering hci_uart_tty_close() to clear HCI_UART_PROTO_READY first, followed immediately by a cancel_work_sync(&hu->write_work). Clearing the flag locks out concurrent protocol timers from successfully invoking hci_uart_tx_wakeup(), effectively rendering the cancellation permanent and preventing the tx_skb double-free.
2. Note: Clearing PROTO_READY early causes hci_uart_close() to skip hu->proto->flush(). This is perfectly safe in the tty_close path because hu->proto->close() executes shortly after, which intrinsically purges all protocol SKB queues and tears down the state.
3. Relocating hu->proto->close(hu) strictly prior to hci_free_dev(hdev) across all close and error paths to prevent vendor-level UAFs.
4. Moving the hdev->stat.byte_rx increment in hci_uart_tty_receive() inside the proto_lock read-side critical section to safely synchronize with device unregistration.
5. Adding cancel_work_sync(&hu->write_work) to hci_uart_close() to safely flush the workqueue before hci_uart_flush() is invoked via the HCI core.
6. Utilizing cancel_work_sync() instead of disable_work_sync() across all paths to prevent permanently breaking user-space retry capabilities.

Remediation

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