netdev - Re: [PATCH] net: Fix double free and memory corruption in get_net_ns_by

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Date:   Wed, 20 Dec 2017 16:00:19 +0100
From:   Nicolas Dichtel <nicolas.dichtel@...nd.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>, netdev@...r.kernel.org
Cc:     David Miller <davem@...emloft.net>, ktkhai@...tuozzo.com,
        security@...nel.org, secalert@...hat.com, eric.dumazet@...il.com,
        stephen@...workplumber.org
Subject: Re: [PATCH] net: Fix double free and memory corruption in
 get_net_ns_by_id()

Le 19/12/2017 à 18:27, Eric W. Biederman a écrit :
> 
> (I can trivially verify that that idr_remove in cleanup_net happens
>  after the network namespace count has dropped to zero --EWB)
> 
> Function get_net_ns_by_id() does not check for net::count
> after it has found a peer in netns_ids idr.
> 
> It may dereference a peer, after its count has already been
> finaly decremented. This leads to double free and memory
> corruption:
> 
> put_net(peer)                                   rtnl_lock()
> atomic_dec_and_test(&peer->count) [count=0]     ...
> __put_net(peer)                                 get_net_ns_by_id(net, id)
>   spin_lock(&cleanup_list_lock)
>   list_add(&net->cleanup_list, &cleanup_list)
>   spin_unlock(&cleanup_list_lock)
> queue_work()                                      peer = idr_find(&net->netns_ids, id)
>   |                                               get_net(peer) [count=1]
>   |                                               ...
>   |                                               (use after final put)
>   v                                               ...
>   cleanup_net()                                   ...
>     spin_lock(&cleanup_list_lock)                 ...
>     list_replace_init(&cleanup_list, ..)          ...
>     spin_unlock(&cleanup_list_lock)               ...
>     ...                                           ...
>     ...                                           put_net(peer)
>     ...                                             atomic_dec_and_test(&peer->count) [count=0]
>     ...                                               spin_lock(&cleanup_list_lock)
>     ...                                               list_add(&net->cleanup_list, &cleanup_list)
>     ...                                               spin_unlock(&cleanup_list_lock)
>     ...                                             queue_work()
>     ...                                           rtnl_unlock()
>     rtnl_lock()                                   ...
>     for_each_net(tmp) {                           ...
>       id = __peernet2id(tmp, peer)                ...
>       spin_lock_irq(&tmp->nsid_lock)              ...
>       idr_remove(&tmp->netns_ids, id)             ...
>       ...                                         ...
>       net_drop_ns()                               ...
> 	net_free(peer)                            ...
>     }                                             ...
>   |
>   v
>   cleanup_net()
>     ...
>     (Second free of peer)
> 
> Also, put_net() on the right cpu may reorder with left's cpu
> list_replace_init(&cleanup_list, ..), and then cleanup_list
> will be corrupted.
> 
> Since cleanup_net() is executed in worker thread, while
> put_net(peer) can happen everywhere, there should be
> enough time for concurrent get_net_ns_by_id() to pick
> the peer up, and the race does not seem to be unlikely.
> The patch fixes the problem in standard way.
> 
> (Also, there is possible problem in peernet2id_alloc(), which requires
> check for net::count under nsid_lock and maybe_get_net(peer), but
> in current stable kernel it's used under rtnl_lock() and it has to be
> safe. Openswitch begun to use peernet2id_alloc(), and possibly it should
> be fixed too. While this is not in stable kernel yet, so I'll send
> a separate message to netdev@ later).
> 
> Cc: Nicolas Dichtel <nicolas.dichtel@...nd.com>
> Signed-off-by: Kirill Tkhai <ktkhai@...tuozzo.com>
> Fixes: 0c7aecd4bde4 "netns: add rtnl cmd to add and get peer netns ids"
> Reviewed-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> Reviewed-by: "Eric W. Biederman" <ebiederm@...ssion.com>
> Signed-off-by: Eric W. Biederman <ebiederm@...ssion.com>
Good catch, thank you.

Acked-by: Nicolas Dichtel <nicolas.dichtel@...nd.com>