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

[Edward Adam Davis]

On Wed, 4 Sep 2024 22:39:10 +0200, Matthieu Baerts wrote:
>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:
Got it, I have added them in V3 patch.
>
>  [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?
I strongly agree. This can avoid accessing the entry outside the lock.
I have integrated your code to my patch.

BR,
Edward