[PATCH net v3] mptcp: fix race in mptcp_pm_nl_flush_addrs_doit()

From: Eric Dumazet <edumazet@google.com>

syzbot and Eulgyu Kim reported crashes in mptcp_pm_nl_get_local_id()
and/or mptcp_pm_nl_is_backup()

Root cause is list_splice_init() in mptcp_pm_nl_flush_addrs_doit()
which is not RCU ready.

list_splice_init_rcu() can not be called here while holding pernet->lock
spinlock.

Many thanks to Eulgyu Kim for providing a repro and testing our patches.

Fixes: 141694df6573 ("mptcp: remove address when netlink flushes addrs")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot+5498a510ff9de39d37da@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/6970a46d.a00a0220.3ad28e.5cf0.GAE@google.com/T/ (local)
Reported-by: Eulgyu Kim <redacted>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/611
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
v3:
 - Same code, but the v2 was corrupted: leading whitespaces before tabs
   had been accidentally removed, causing 'git am' to reject the patch.
 - Add another Closes tag to MPTCP bug tracker.
 - Remove empty lines after 'spin_lock_bh()' to keep the same style.
v2:
 - Make sure the list was not empty, return early otherwise.
 - https://lore.kernel.org/20260123030327.3041148-1-edumazet@google.com (local)
v1:
 - https://lore.kernel.org/20260122131306.2119853-1-edumazet@google.com (local)
---
 net/mptcp/pm_kernel.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index 57570a44e418..b26675054b0d 100644
--- a/net/mptcp/pm_kernel.c
+++ b/net/mptcp/pm_kernel.c

@@ -1294,16 +1294,26 @@ static void __reset_counters(struct pm_nl_pernet *pernet)
 int mptcp_pm_nl_flush_addrs_doit(struct sk_buff *skb, struct genl_info *info)
 {
 	struct pm_nl_pernet *pernet = genl_info_pm_nl(info);
-	LIST_HEAD(free_list);
+	struct list_head free_list;
 
 	spin_lock_bh(&pernet->lock);
-	list_splice_init(&pernet->endp_list, &free_list);
+	free_list = pernet->endp_list;
+	INIT_LIST_HEAD_RCU(&pernet->endp_list);
 	__reset_counters(pernet);
 	pernet->next_id = 1;
 	bitmap_zero(pernet->id_bitmap, MPTCP_PM_MAX_ADDR_ID + 1);
 	spin_unlock_bh(&pernet->lock);
-	mptcp_nl_flush_addrs_list(sock_net(skb->sk), &free_list);
+
+	if (free_list.next == &pernet->endp_list)
+		return 0;
+
 	synchronize_rcu();
+
+	/* Adjust the pointers to free_list instead of pernet->endp_list */
+	free_list.prev->next = &free_list;
+	free_list.next->prev = &free_list;
+
+	mptcp_nl_flush_addrs_list(sock_net(skb->sk), &free_list);
 	__flush_addrs(&free_list);
 	return 0;
 }

---
base-commit: 8016dc5ee19a77678c264f8ba368b1e873fa705b
change-id: 20260124-net-mptcp-race_nl_flush_addrs-337f460d1c9a

Best regards,
-- 
Matthieu Baerts (NGI0) [off-list ref]