[PATCH 6.18 423/752] Bluetooth: L2CAP: Fix not checking output MTU is acceptable on L2CAP_ECRED_CONN_REQ

Sasha Levin
2026-02-28T17:54:02Z

From: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

[ Upstream commit a8d1d73c81d1e70d2aa49fdaf59d933bb783ffe5 ]

Upon receiving L2CAP_ECRED_CONN_REQ the given MTU shall be checked
against the suggested MTU of the listening socket as that is required
by the likes of PTS L2CAP/ECFC/BV-27-C test which expects
L2CAP_CR_LE_UNACCEPT_PARAMS if the MTU is lowers than socket omtu.

In order to be able to set chan->omtu the code now allows setting
setsockopt(BT_SNDMTU), but it is only allowed when connection has not
been stablished since there is no procedure to reconfigure the output
MTU.

Link: https://github.com/bluez/bluez/issues/1895
Fixes: 15f02b910562 ("Bluetooth: L2CAP: Add initial code for Enhanced Credit Based Mode")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/bluetooth/l2cap_core.c | 8 ++++++++
net/bluetooth/l2cap_sock.c | 15 +++++++++++----
2 files changed, 19 insertions(+), 4 deletions(-)

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index a5038160675ea..29af3f63e89ce 100644
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -5101,6 +5101,14 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
goto unlock;
}

+ /* Check if the listening channel has set an output MTU then the
+ * requested MTU shall be less than or equal to that value.
+ */
+ if (pchan->omtu && mtu < pchan->omtu) {
+ result = L2CAP_CR_LE_UNACCEPT_PARAMS;
+ goto unlock;
+ }
+
result = L2CAP_CR_LE_SUCCESS;

for (i = 0; i < num_scid; i++) {
diff --git a/net/bluetooth/l2cap_sock.c b/net/bluetooth/l2cap_sock.c
index 814fb8610ac43..c877fe5aed07d 100644
--- a/net/bluetooth/l2cap_sock.c
+++ b/net/bluetooth/l2cap_sock.c
@@ -1029,10 +1029,17 @@ static int l2cap_sock_setsockopt(struct socket *sock, int level, int optname,
break;
}

- /* Setting is not supported as it's the remote side that
- * decides this.
- */
- err = -EPERM;
+ /* Only allow setting output MTU when not connected */
+ if (sk->sk_state == BT_CONNECTED) {
+ err = -EISCONN;
+ break;
+ }
+
+ err = copy_safe_from_sockptr(&mtu, sizeof(mtu), optval, optlen);
+ if (err)
+ break;
+
+ chan->omtu = mtu;
break;

case BT_RCVMTU:
--
2.51.0