GHSA-ffxg-752w-mv9x: Bluetooth Classic AVDTP session reuse re-initializes a queued work item, corrupting the system work queue

The Bluetooth Classic AVDTP layer defers releasing a session's stream endpoints: btavdtpl2capdisconnected() in subsys/bluetooth/host/classic/avdtp.c ends with kworksubmit(&session->releasework), placing the embedded struct kwork on the system work queue's pending list. When the same struct btavdtp object is immediately reused for a new signalling connection, the session memory is re-initialized — a2dpgetconnection() in subsys/bluetooth/host/classic/a2dp.c memset()s the whole struct bta2dp, and btavdtpl2capaccept() / btavdtpconnect() call kworkinit() on session->releasework — while that work item is still linked into ksysworkq.pending. Zeroing a linked syssnodet truncates the list while its tail pointer keeps pointing past the truncation.

The reuse window is deterministic rather than a race. All host RX processing runs on the Bluetooth RX work queue at a cooperative priority (CONFIGBTRXPRIO), while releasework is submitted to the lower-priority system work queue, so the release handler cannot run until the RX thread blocks. Because l2capbrrecv() processes every signalling command contained in a single L2CAP signalling PDU back-to-back, a peer can send one PDU holding an L2CAP Disconnection Request for the AVDTP signalling channel followed by a Connection Request for PSM 0x0019; the second command reaches the session-reuse path with the release work provably still queued. The peer must already hold an encrypted AVDTP signalling channel, i.e. be a paired/bonded BR/EDR device.

The result is corruption of the global system work queue's pending list: either the queue silently stops executing any work at all (subsequent submissions are appended to a chain the head can no longer reach, and the orphaned items keep their KWORKQUEUED flag forever) or, in the tail variant, the list acquires a self-referencing node that the queue thread spins on. The written values are pointers to other struct kwork objects and are not attacker-controlled, so the impact is a device-wide, reboot-required denial of service rather than a controllable write primitive. Only builds with CONFIGBTCLASSIC and CONFIGBTA2DP are affected.

The fix is a two-commit unit, and neither commit closes the path on its own. ec822ceed83b adds a pending-release check to btavdtpconnect() and btavdtpl2capaccept(), but the A2DP upper layer's a2dpgetconnection() memset()s the session — including the queued work item's list node and flags — before those checks run, so kworkbusyget() would observe an already-cleared state. 2bd5ea88c69b closes that by making a2dpgetconnection() itself skip the reuse and return NULL while the release work is still pending, which both callers already handle by rejecting the connection attempt. Apply both commits together; either one alone leaves the device vulnerable.