Re: [PATCH net] mptcp: fix race in mptcp_pm_nl_flush_addrs_doit()
From: Eric Dumazet <edumazet@google.com>
Date: 2026-01-22 13:54:59
Subsystem:
networking [general], networking [mptcp], the rest · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Matthieu Baerts, Mat Martineau, Linus Torvalds
On Thu, Jan 22, 2026 at 2:13 PM Eric Dumazet [off-list ref] wrote:
quoted hunk ↗ jump to hunk
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> 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;
We have to test if the list was empty, and avoid the synchronize_rcu in this case. I will squash in V2, unless someone complains.
diff --git a/net/mptcp/pm_kernel.c b/net/mptcp/pm_kernel.c
index 1a97d0eafa2b0c9e4275b90d4a576f837dc286a9..af23be6658ded4860133bb9495c7738014815d28100644
--- a/net/mptcp/pm_kernel.c
+++ b/net/mptcp/pm_kernel.c@@ -1305,6 +1305,10 @@ int mptcp_pm_nl_flush_addrs_doit(struct sk_buff*skb, struct genl_info *info)
pernet->next_id = 1;
bitmap_zero(pernet->id_bitmap, MPTCP_PM_MAX_ADDR_ID + 1);
spin_unlock_bh(&pernet->lock);
+
+ if (free_list.next == &pernet->endp_list)
+ return 0;
+
synchronize_rcu();
/* Adjust the pointers to free_list instead of pernet->endp_list */