mptcp: fix race in mptcp_pm_nl_flush_addrs_doit()

From: Eric Dumazet <[email protected]>

commit e2a9eeb69f7d4ca4cf4c70463af77664fdb6ab1d upstream.

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.

[ Conflicts because the code has been moved from pm_netlink.c to
pm_kernel.c later on in commit 8617e85e04bd ("mptcp: pm: split in-kernel PM specific code"). The same modifications can be applied in pm_netlink.c with one exception, because 'pernet->local_addr_list' has been renamed to 'pernet->endp_list' in commit 35e71e43a56d ("mptcp: pm: in-kernel: rename 'local_addr_list' to 'endp_list'"). The previous name is then still being used in this version. Also, another conflict is caused by commit 7bcf4d8022f9 ("mptcp: pm:
rename helpers linked to 'flush'") which is not in this version:
mptcp_nl_remove_addrs_list() has been renamed to mptcp_nl_flush_addrs_list(). The previous name has then been kept. ]

---
 net/mptcp/pm_netlink.c | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

index 9f25cfd96f98..4b805d7f5769 100644
@@ -1859,16 +1859,26 @@ static void __reset_counters(struct pm_nl_pernet *pernet)
 static int mptcp_nl_cmd_flush_addrs(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->local_addr_list, &free_list);
+	free_list = pernet->local_addr_list;
+	INIT_LIST_HEAD_RCU(&pernet->local_addr_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_remove_addrs_list(sock_net(skb->sk), &free_list);
+
+	if (free_list.next == &pernet->local_addr_list)
+		return 0;
+
 	synchronize_rcu();
+
+	/* Adjust the pointers to free_list instead of pernet->local_addr_list */
+	free_list.prev->next = &free_list;
+	free_list.next->prev = &free_list;
+
+	mptcp_nl_remove_addrs_list(sock_net(skb->sk), &free_list);
 	__flush_addrs(&free_list);
 	return 0;
 }

2.51.0