[PATCH 6.6 24/25] mptcp: fix race in mptcp_pm_nl_flush_addrs_doit()

Greg Kroah-Hartman
2026-02-13T13:57:24Z

From: Eric Dumazet <edumazet@google.com>

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.

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>
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>
Link: https://patch.msgid.link/20260124-net-mptcp-race_nl_flush_addrs-v3-1-b2dc1b613e9d@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ 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. ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/mptcp/pm_netlink.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

--- a/net/mptcp/pm_netlink.c
+++ b/net/mptcp/pm_netlink.c
@@ -1859,16 +1859,26 @@ static void __reset_counters(struct pm_n
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;
}