CVE-2026-19938: Use-after-free of an ATT PDU buffer when signing an outgoing Bluetooth ATT Signed Write Command fails

The Bluetooth host's ATT transmit path releases a buffer it does not own when signing fails. chansend() in subsys/bluetooth/host/att.c documents (and every other error exit implements) the contract that on failure the caller retains ownership of the netbuf; the BTATTOPSIGNEDWRITECMD branch instead called netbufunref(buf) when btsmpsign() returned an error and then returned that error to the caller. The callers act on the contract: processqueue() re-queues the buffer with kqueueprepend(), btattchansendrsp() puts it back on chan->txqueue, btattchanreqsend() restores it into the request, and attreqretry() unreferences it — so the already-freed buffer was re-queued, later dereferenced, and eventually freed again.

An ATT Signed Write Command is produced when an application calls btgattwritewithoutresponsecb() with sign set on an unencrypted connection. btsmpsign() fails whenever no local CSRK exists for the peer (-ENOENT), and whether one exists is negotiated with the peer: smpdistributekeys() stores a local CSRK only when the peer's pairing key-distribution field requests the signing key and bonding was agreed. A peer can therefore pair without accepting the CSRK, pair with No Bonding, remain unpaired, or keep the link unencrypted, and the victim's next signed write takes the failing path. Because struct netbuf's node field is simultaneously the linkage word for the ATT TX FIFO, for the deferred-destroy list used by the attpool destroy callback atttxdestroy(), and for the pool free list, the mishandled buffer corrupts these lists and can be re-issued by btattcreatepdu() to a new PDU while the ATT TX path still holds it.

The same branch also saved the netbufsimplestate only after signing, so a buffer restored after a btl2capsendpdu() failure kept its 12-octet signature; a subsequent retry re-signed it and appended a second signature with netbufadd(buf, 12) to a buffer that btattcreatepdu() sized for exactly one, writing past the end of the pooled buffer in builds where the netbuf tailroom assertion is compiled out.

Impact is host-side memory corruption in the Bluetooth stack: list corruption, a pooled ATT buffer with two owners (wrong PDU contents transmitted, or a double free when the queues are drained at disconnect), and a bounded out-of-bounds write on the retry path — typically observed as a crash or assertion failure. Exposure requires the deprecated CONFIGBTSIGNING to be enabled and an application that actually issues signed writes; in that configuration the triggering condition is under the remote peer's control.