CVE-2026-64127

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).