Bluetooth: L2CAP: ecred_reconfigure: send packed pdu, not stack pointer

Overview

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

Bluetooth: L2CAP: ecred_reconfigure: send packed pdu, not stack pointer

Commit 1c08108f3014 ("Bluetooth: L2CAP: Avoid -Wflex-array-member-not-at-end warnings") converted the on-stack request PDU in l2cap_ecred_reconfigure() from an explicit packed struct to DEFINE_RAW_FLEX(), but did not adjust the size and source-pointer arguments to l2cap_send_cmd():

• struct {

•        struct l2cap_ecred_reconf_req req;
  

•        __le16 scid;
  

• } pdu;

• DEFINE_RAW_FLEX(struct l2cap_ecred_reconf_req, pdu, scid, 1); ... l2cap_send_cmd(conn, chan->ident, L2CAP_ECRED_RECONF_REQ, sizeof(pdu), &pdu);

After the conversion, DEFINE_RAW_FLEX() expands to declare an anonymous union pdu_u plus a local pointer "pdu" pointing at it. Therefore:

• sizeof(pdu) is now sizeof(struct l2cap_ecred_reconf_req *) = 8 on 64-bit (4 on 32-bit), not the 6 bytes of (mtu, mps, scid[1]).
• &pdu is the address of the local pointer's stack storage, not the address of the request payload.

l2cap_send_cmd() forwards (data, count) to l2cap_build_cmd(), which calls skb_put_data(skb, data, count). The L2CAP_ECRED_RECONFIGURE_REQ packet body therefore contains 8 bytes copied from the kernel stack starting at &pdu -- the 8 bytes overlap the pdu pointer's value, leaking a kernel stack address to the paired Bluetooth peer. The intended (mtu, mps, scid) fields are not transmitted at all, so the peer rejects the request as malformed and the L2CAP_ECRED_RECONFIGURE feature itself has been broken for the local-side initiator since the introducing commit landed.

The sibling site l2cap_ecred_conn_req() in the same commit was converted correctly (sizeof(*pdu) + len, pdu); only this site was missed.

Restore the original semantics: pass the full flex-struct size via struct_size(pdu, scid, 1) and the pdu pointer (the struct address) as the source.

Validated on a stock 7.0-based host kernel via the real call path: setsockopt(SOL_BLUETOOTH, BT_RCVMTU, ...) on a BT_CONNECTED L2CAP_MODE_EXT_FLOWCTL socket emits an L2CAP_ECRED_RECONFIGURE_REQ whose body is 8 bytes (the on-stack pdu local's value) rather than the expected 6. Three captures from fresh socket / fresh hciemu peer on the same host -- low bytes vary per call, high 0xffff confirms a kernel virtual address (KASLR-randomised stack slot, not a fixed string):

RECONF_REQ body (ident=0x02 len=8): 42 fb 54 af 0e ca ff ff RECONF_REQ body (ident=0x02 len=8): 52 3d 2e af 0e ca ff ff RECONF_REQ body (ident=0x02 len=8): b2 fc 5b af 0e ca ff ff

After this patch the body is 6 bytes carrying the expected little-endian (mtu, mps, scid).

Affected

• Linux >= 1c08108f3014881ad5f4c35a2abaf9c65475035d < ed5fcd2a26f0c16fc289c8cd6b03328a0582a687
• Linux >= 1c08108f3014881ad5f4c35a2abaf9c65475035d < 051922ab709c0a6917eae765c22481dfc68379e5
• Linux >= 1c08108f3014881ad5f4c35a2abaf9c65475035d < 356c9d1a1cbacd2a1640fff3c050e1c3472e924f
• Linux >= 1c08108f3014881ad5f4c35a2abaf9c65475035d < 3374ef8cf99368a40f7efd51a2a375a4c5dc6f0d
• Linux 6.10

Remediation

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