Re: [PATCH V2] mptcp: pm: Fix uaf in __timer_delete_sync

[Matthieu Baerts]

Hello,

Thank you for this patch!

On 04/09/2024 03:01, Edward Adam Davis 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

Please add a Fixes tag and Cc stable.

And add 'net' after PATCH in the subject:

  [PATCH net v3]

> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---
>  net/mptcp/pm_netlink.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/mptcp/pm_netlink.c b/net/mptcp/pm_netlink.c
> index 3e4ad801786f..d4cbf7dcf983 100644
> --- a/net/mptcp/pm_netlink.c
> +++ b/net/mptcp/pm_netlink.c
> @@ -1430,8 +1430,10 @@ static bool remove_anno_list_by_saddr(struct mptcp_sock *msk,
>  
>  	entry = mptcp_pm_del_add_timer(msk, addr, false);
>  	if (entry) {
> +		spin_lock_bh(&msk->pm.lock);
>  		list_del(&entry->list);
>  		kfree(entry);
> +		spin_unlock_bh(&msk->pm.lock);

Mmh, I can understand it would help to reduce issues here, but I don't
think that's enough: in mptcp_pm_del_add_timer(), CPU1 can get the entry
from the list under the lock, then immediately after, the free can
happen on CPU2, while CPU1 is trying to access entry->add_timer outside
the lock, no? Something like this:

  CPU1              CPU2
  ====              ====
  entry = (...)
                    kfree(entry)
  entry->add_timer


What about keeping a reference to add_timer inside the lock, and calling
sk_stop_timer_sync() with this reference, instead of "entry->add_timer"?
I'm thinking about something like that to be applied *on top* of your
patch, WDYT?

  https://lore.kernel.org/20240904170517.237863-2-matttbe@xxxxxxxxxx

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.