From b878dfdd12d7a5b8722a78d35e313506140ca3d9 Mon Sep 17 00:00:00 2001 From: Harshit Varu Date: Sat, 15 Aug 2026 17:22:05 +0530 Subject: [PATCH] mptcp: fix uninitialized local_id in syncookie MP_JOIN reconstruction mptcp_token_join_cookie_init_state() restores remote_nonce, local_nonce, backup, join_id, token and msk from the saved cookie entry when rebuilding the request socket for a MP_JOIN 4th-ACK handled under SYN cookies, but it does not restore local_id, even though the SYN path saved it. subflow_ulp_clone() then reads that uninitialized field and stores it as the joined subflow's address-ID. Because the request-sock slab is SLAB_TYPESAFE_BY_RCU and not zeroed on allocation, the value is the stale byte of a previously freed request socket, which an off-path peer can influence by sending concurrent MP_JOIN SYNs. This corrupts the path manager's id-based subflow bookkeeping for the connection. Restore subflow_req->local_id from the cookie entry, as done for the other fields. Fixes: 9466a1ccebbe ("mptcp: enable JOIN requests even if cookies are in use") Cc: stable@vger.kernel.org Signed-off-by: Harshit Varu Reviewed-by: Matthieu Baerts (NGI0) Link: https://patch.msgid.link/20260815115205.197151-1-harshitvaru666@gmail.com Signed-off-by: Jakub Kicinski --- net/mptcp/syncookies.c | 1 + 1 file changed, 1 insertion(+) diff --git a/net/mptcp/syncookies.c b/net/mptcp/syncookies.c index 7f22526346a7..b5cac5701122 100644 --- a/net/mptcp/syncookies.c +++ b/net/mptcp/syncookies.c @@ -118,6 +118,7 @@ bool mptcp_token_join_cookie_init_state(struct mptcp_subflow_request_sock *subfl subflow_req->local_nonce = e->local_nonce; subflow_req->backup = e->backup; subflow_req->remote_id = e->join_id; + subflow_req->local_id = e->local_id; subflow_req->token = e->token; subflow_req->msk = msk; spin_unlock_bh(&join_entry_locks[i]);