Bluetooth: L2CAP: Fix stack-out-of-bounds read in l2cap_ecred_conn_req

Syzbot reported a KASAN stack-out-of-bounds read in l2cap_build_cmd()
that is triggered by a malformed Enhanced Credit Based Connection Request.

The vulnerability stems from l2cap_ecred_conn_req(). The function allocates
a local stack buffer (`pdu`) designed to hold a maximum of 5 Source Channel
IDs (SCIDs), totaling 18 bytes. When an attacker sends a request with more
than 5 SCIDs, the function calculates `rsp_len` based on this unvalidated
`cmd_len` before checking if the number of SCIDs exceeds
L2CAP_ECRED_MAX_CID.

If the SCID count is too high, the function correctly jumps to the
`response` label to reject the packet, but `rsp_len` retains the
attacker's oversized value. Consequently, l2cap_send_cmd() is instructed
to read past the end of the 18-byte `pdu` buffer, triggering a
KASAN panic.

Fix this by moving the assignment of `rsp_len` to after the `num_scid`
boundary check. If the packet is rejected, `rsp_len` will safely
remain 0, and the error response will only read the 8-byte base header
from the stack.

Fixes: c28d2bff7044 ("Bluetooth: L2CAP: Fix result of L2CAP_ECRED_CONN_RSP when MTU is too short")
Reported-by: syzbot+b7f3e7d9a596bf6a63e3@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=b7f3e7d9a596bf6a63e3
Tested-by: syzbot+b7f3e7d9a596bf6a63e3@syzkaller.appspotmail.com
Signed-off-by: Minseo Park <jacob.park.9436@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

diff --git a/net/bluetooth/l2cap_core.c b/net/bluetooth/l2cap_core.c
index 5deb6c4f1e41da74d8035c5ca74f99e7fcd9d1d3..0882b5ac2ecca646faf8f132ea81a9c6ddb4eccd 100644 (file)
--- a/net/bluetooth/l2cap_core.c
+++ b/net/bluetooth/l2cap_core.c
@@ -5081,14 +5081,14 @@ static inline int l2cap_ecred_conn_req(struct l2cap_conn *conn,
        cmd_len -= sizeof(*req);
        num_scid = cmd_len / sizeof(u16);
 
-       /* Always respond with the same number of scids as in the request */
-       rsp_len = cmd_len;
-
        if (num_scid > L2CAP_ECRED_MAX_CID) {
                result = L2CAP_CR_LE_INVALID_PARAMS;
                goto response;
        }
 
+       /* Always respond with the same number of scids as in the request */
+       rsp_len = cmd_len;
+
        mtu  = __le16_to_cpu(req->mtu);
        mps  = __le16_to_cpu(req->mps);