Re: [PATCH] mptcp: pm: Fix uaf in __timer_delete_sync

[Eric Dumazet]

On Tue, Sep 3, 2024 at 5:10 PM Edward Adam Davis <eadavis@xxxxxx> wrote:
>
> There are two paths to access mptcp_pm_del_add_timer, result in a race
> condition:
>
>      CPU1                               CPU2
>      ====                               ====
>      net_rx_action
>      napi_poll                          netlink_sendmsg
>      __napi_poll                        netlink_unicast
>      process_backlog                    netlink_unicast_kernel
>      __netif_receive_skb                genl_rcv
>      __netif_receive_skb_one_core       netlink_rcv_skb
>      NF_HOOK                            genl_rcv_msg
>      ip_local_deliver_finish            genl_family_rcv_msg
>      ip_protocol_deliver_rcu            genl_family_rcv_msg_doit
>      tcp_v4_rcv                         mptcp_pm_nl_flush_addrs_doit
>      tcp_v4_do_rcv                      mptcp_nl_remove_addrs_list
>      tcp_rcv_established                mptcp_pm_remove_addrs_and_subflows
>      tcp_data_queue                     remove_anno_list_by_saddr
>      mptcp_incoming_options             mptcp_pm_del_add_timer
>      mptcp_pm_del_add_timer             kfree(entry)
>
> In remove_anno_list_by_saddr(running on CPU2), after leaving the critical
> zone protected by "pm.lock", the entry will be released, which leads to the
> occurrence of uaf in the mptcp_pm_del_add_timer(running on CPU1).
>
> Reported-and-tested-by: syzbot+f3a31fb909db9b2a5c4d@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=f3a31fb909db9b2a5c4d
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---
>  net/mptcp/pm_netlink.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
> index 3e4ad801786f..d28bf0c9ad66 100644
> --- a/net/mptcp/pm_netlink.c
> +++ b/net/mptcp/pm_netlink.c
> @@ -336,11 +336,12 @@ mptcp_pm_del_add_timer(struct mptcp_sock *msk,
>         entry = mptcp_lookup_anno_list_by_saddr(msk, addr);
>         if (entry && (!check_id || entry->addr.id == addr->id))
>                 entry->retrans_times = ADD_ADDR_RETRANS_MAX;
> -       spin_unlock_bh(&msk->pm.lock);
>
>         if (entry && (!check_id || entry->addr.id == addr->id))
>                 sk_stop_timer_sync(sk, &entry->add_timer);
>
> +       spin_unlock_bh(&msk->pm.lock);


mptcp_pm_add_timer() needs to lock msk->pm.lock

Your patch might add a deadlock, because sk_stop_timer_sync() is
calling del_timer_sync()

What is preventing this ?