[PATCH] Bluetooth: L2CAP: ecred_reconfigure: send packed pdu, not stack pointer

[Michael Bommarito <michael.bommarito@gmail.com>]

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).

Cc: stable@vger.kernel.org
Fixes: 1c08108f3014 ("Bluetooth: L2CAP: Avoid -Wflex-array-member-not-at-end warnings")
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
---
 net/bluetooth/l2cap_core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 77dec104a9c3..4773a453b145 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -7282,7 +7282,7 @@ static void l2cap_ecred_reconfigure(struct l2cap_chan *chan)
 	chan->ident = l2cap_get_ident(conn);
 
 	l2cap_send_cmd(conn, chan->ident, L2CAP_ECRED_RECONF_REQ,
-		       sizeof(pdu), &pdu);
+		       struct_size(pdu, scid, 1), pdu);
 }
 
 int l2cap_chan_reconfigure(struct l2cap_chan *chan, __u16 mtu)
-- 
2.53.0