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linux/include/net/bluetooth
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Michael Bommarito fa58231262 Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig 
commit dd21473354 upstream.

net/bluetooth/l2cap_core.c:l2cap_sig_channel() accepts BR/EDR
signaling packets up to the channel MTU and dispatches each command
without enforcing the signaling MTU (MTUsig). A Bluetooth BR/EDR peer
within radio range can send a fixed-channel CID 0x0001 packet that is
larger than MTUsig and contains many L2CAP_ECHO_REQ commands before
pairing. In a real-radio stock-kernel run, one 681-byte signaling
packet containing 168 zero-length ECHO_REQ commands made the target
transmit 168 ECHO_RSP frames over about 220 ms.

Impact: a Bluetooth BR/EDR peer within radio range, before pairing, can
force 168 ECHO_RSP frames from one 681-byte fixed-channel signaling
packet containing packed ECHO_REQ commands.

Define Linux's BR/EDR signaling MTU as the spec minimum of 48 bytes and
reject any larger signaling packet with one L2CAP_COMMAND_REJECT_RSP
carrying L2CAP_REJ_MTU_EXCEEDED before any command is dispatched.

The Bluetooth Core spec wording for MTUExceeded says the reject
identifier shall match the first request command in the packet, and
that packets containing only responses shall be silently discarded.
Linux intentionally deviates from that prescription: silently
discarding desynchronizes the peer because the remote stack never
learns its responses were dropped, and locating the first request
command requires walking command headers past MTUsig, i.e. processing
bytes from a packet we have already decided is too large to process.
We therefore always emit one reject and use the identifier from the
first command header, a single fixed-offset byte read.

The unrestricted BR/EDR signaling parser and ECHO_REQ response path both
trace to the initial git import; no later introducing commit is
available for a Fixes tag.

Cc: stable@vger.kernel.org
Suggested-by: Luiz Augusto von Dentz <luiz.dentz@gmail.com>
Link: https://lore.kernel.org/r/20260518002800.1361430-1-michael.bommarito@gmail.com
Link: https://lore.kernel.org/r/20260520135034.1060859-1-michael.bommarito@gmail.com
Link: https://lore.kernel.org/r/20260521000555.3712030-1-michael.bommarito@gmail.com
Assisted-by: Claude:claude-opus-4-7
Assisted-by: Codex:gpt-5-5-xhigh
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
History 2026-06-19 13:33:36 +02:00
..
bluetooth.h Bluetooth: SCO: Fix UAF on sco_sock_timeout 2025-05-02 07:44:25 +02:00
hci.h Bluetooth: add quirk disabling LE Read Transmit Power 2022-01-16 09:12:44 +01:00
hci_core.h Bluetooth: hci_core: Fix use-after-free in vhci_flush() 2026-06-01 17:35:09 +02:00
hci_mon.h Bluetooth: hci_sock: Correctly bounds check and pad HCI_MON_NEW_INDEX name 2023-10-25 11:59:04 +02:00
hci_sock.h Bluetooth: Use only 8 bits for the HCI CMSG state flags 2020-06-12 15:10:46 +02:00
l2cap.h Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig 2026-06-19 13:33:36 +02:00
mgmt.h Bluetooth: use inclusive language in SMP 2021-06-26 07:12:37 +02:00
rfcomm.h Bluetooth: Replace zero-length array with flexible-array member 2020-02-28 08:30:02 +01:00
sco.h Bluetooth: Add support for BT_PKT_STATUS CMSG data for SCO connections 2020-06-12 15:08:49 +02:00