[PATCH] mptcp: serialize subflow->closing with RX path

[Kalpan Jani <kalpan.jani@mpiricsoftware.com>]

There is a race between mptcp_data_ready() (RX path) and
mptcp_close_ssk() (teardown path) when accessing subflow->closing.

Currently, mptcp_data_ready() checks subflow->closing before acquiring
mptcp_data_lock(), while mptcp_close_ssk() may concurrently set
subflow->closing and purge backlog entries. This creates a classic
time-of-check vs time-of-use (TOCTOU) race:

  CPU A (close path)              CPU B (RX path)
  ----------------------         -------------------------
  set closing = 1
                                 read closing == 0
  purge backlog
                                 enqueue skb to backlog

As a result, skb entries referencing the subflow socket (ssk) may be
enqueued after the subflow is marked closing and scheduled for cleanup.
This can lead to:

  - WARN in inet_sock_destruct() due to non-zero sk_rmem_alloc
  - potential use-after-free via stale skb->sk references

Fix this by serializing both the closing check and backlog enqueue
under mptcp_data_lock(). This ensures that subflow->closing state and
backlog operations are observed atomically, preventing new skb from
being enqueued once teardown begins.

Also protect backlog cleanup in mptcp_close_ssk() with the same lock
to guarantee mutual exclusion with the RX path.

This restores proper synchronization between RX and teardown paths
and prevents stale skb references to closing subflows.

Signed-off-by: Kalpan Jani <kalpan.jani@mpiricsoftware.com>
---
 net/mptcp/protocol.c | 31 ++++++++++++++++++++++++++++---
 1 file changed, 28 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 718e910ff..295f8e1c0 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -910,14 +910,34 @@ void mptcp_data_ready(struct sock *sk, struct sock *ssk)
 	struct mptcp_subflow_context *subflow = mptcp_subflow_ctx(ssk);
 	struct mptcp_sock *msk = mptcp_sk(sk);
 
+	/*
+	 * The close path can set subflow->closing while we are racing
+	 * from BH context here. The old check was done before taking
+	 * mptcp_data_lock(), leaving a TOCTOU window:
+	 *
+	 *   CPU A: close path sets closing = 1 and purges backlog
+	 *   CPU B: already observed closing == 0 and later enqueues skb
+	 *
+	 * That skb keeps skb->sk == ssk and can later trigger:
+	 * - WARN in inet_sock_destruct() (ssk->sk_rmem_alloc != 0)
+	 * - UAF in backlog purge via stale skb->sk
+	 */
+
 	/* The peer can send data while we are shutting down this
 	 * subflow at subflow destruction time, but we must avoid enqueuing
 	 * more data to the msk receive queue
 	 */
-	if (unlikely(subflow->closing))
-		return;
 
 	mptcp_data_lock(sk);
+
+	/* Serialize closing check with backlog enqueue */
+	if (unlikely(subflow->closing)) {
+		mptcp_data_unlock(sk);
+		return;
+	}
+
 	mptcp_rcv_rtt_update(msk, subflow);
 	if (!sock_owned_by_user(sk)) {
 		/* Wake-up the reader only for in-sequence data */
@@ -2653,9 +2673,12 @@ void mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 	if (sk->sk_state == TCP_ESTABLISHED)
 		mptcp_event(MPTCP_EVENT_SUB_CLOSED, mptcp_sk(sk), ssk, GFP_KERNEL);
 
-	/* Remove any reference from the backlog to this ssk; backlog skbs consume
+	/* Remove any reference from the backlog to this ssk.
+	 * Serialize cleanup with RX-side enqueue using mptcp_data_lock().
+	 * Backlog skbs consume
 	 * space in the msk receive queue, no need to touch sk->sk_rmem_alloc
 	 */
+	mptcp_data_lock(sk);
 	list_for_each_entry(skb, &msk->backlog_list, list) {
 		if (skb->sk != ssk)
 			continue;
@@ -2663,6 +2686,8 @@ void mptcp_close_ssk(struct sock *sk, struct sock *ssk,
 		atomic_sub(skb->truesize, &skb->sk->sk_rmem_alloc);
 		skb->sk = NULL;
 	}
+	mptcp_data_unlock(sk);
+
 
 	/* subflow aborted before reaching the fully_established status
 	 * attempt the creation of the next subflow
-- 
2.43.0