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

[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/
Reported-by: Eulgyu Kim <eulgyukim@snu.ac.kr>
Cc: Geliang Tang <geliang@kernel.org>
---
 net/mptcp/pm_kernel.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index 57570a44e4185370f531047fe97ce9f9fbd1480b..1a97d0eafa2b0c9e4275b90d4a576f837dc286a9 100644
--- a/net/mptcp/pm_kernel.c
+++ b/net/mptcp/pm_kernel.c
@@ -1294,16 +1294,24 @@ 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);
 	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;
 }
-- 
2.52.0.457.g6b5491de43-goog