Subject: Re: [PATCH net] ax25: use after free in ax25_connect
From:   Hangyu Hua <hbh25y@gmail.com>
To:     Eric Dumazet <eric.dumazet@gmail.com>, jreuter@yaina.de,
        ralf@linux-mips.org, davem@davemloft.net, kuba@kernel.org
Cc:     linux-hams@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20220111042048.43532-1-hbh25y@gmail.com>
 <f35292c0-621f-3f07-87ed-2533bfd1496e@gmail.com>
 <f48850cb-8e26-afa0-576c-691bb4be5587@gmail.com>
 <571c72e8-2111-6aa0-1bd7-e0af7fc50539@gmail.com>
 <80007b3e-eba8-1fbe-302d-4398830843dd@gmail.com>
Message-ID: <ff65d70b-b6e1-3b35-8bd0-92f6f022cd5d@gmail.com>
Date:   Fri, 14 Jan 2022 14:54:31 +0800
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Any suggestions for this patch ? Guys.

I think putting sk_to_ax25 after lock_sock(sk) here will avoid any 
possilbe race conditions like other functions in ax25_proto_ops.

On 2022/1/12 下午7:11, Hangyu Hua wrote:
> Yes.
> 
> And there are two ways to release ax25, ax25_release and time expiry. I 
> tested that ax25_release will not be invoked before ax25_connect is done 
> by closing fd from user space. I think the reason is that __sys_connect 
> use fdget() to protect fd. But i can't test if a function like 
> ax25_std_heartbeat_expiry will release ax25 between sk_to_ax25(sk) and 
> lock_sock(sk).
> 
> So i think it's better to protect sk_to_ax25(sk) by a lock. Beacause 
> functions like ax25_release use sk_to_ax25 after a lock.
> 
> 
> On 2022/1/12 下午5:59, Eric Dumazet wrote:
>>
>> On 1/11/22 18:13, Hangyu Hua wrote:
>>> I try to use ax25_release to trigger this bug like this:
>>> ax25_release                 ax25_connect
>>> lock_sock(sk);
>>> -----------------------------sk = sock->sk;
>>> -----------------------------ax25 = sk_to_ax25(sk);
>>> ax25_destroy_socket(ax25);
>>> release_sock(sk);
>>> -----------------------------lock_sock(sk);
>>> -----------------------------use ax25 again
>>>
>>> But i failed beacause their have large speed difference. And i
>>> don't have a physical device to test other function in ax25.
>>> Anyway, i still think there will have a function to trigger this
>>> race condition like ax25_destroy_timer. Beacause Any ohter
>>> functions in ax25_proto_ops like ax25_bind protect ax25_sock by 
>>> lock_sock(sk).
>>
>>
>> For a given sk pointer, sk_to_ax25(sk) is always returning the same 
>> value,
>>
>> regardless of sk lock being held or not.
>>
>> ax25_sk(sk)->cb  is set only from ax25_create() or ax25_make_new()
>>
>> ax25_connect can not be called until these operations have completed ?
>>
>>
>>
>>>
>>> Thanks.
>>>
>>>
>>>
>>>
>>> On 2022/1/12 上午4:56, Eric Dumazet wrote:
>>>>
>>>> On 1/10/22 20:20, Hangyu Hua wrote:
>>>>> sk_to_ax25(sk) needs to be called after lock_sock(sk) to avoid UAF
>>>>> caused by a race condition.
>>>>
>>>> Can you describe what race condition you have found exactly ?
>>>>
>>>> sk pointer can not change.
>>>>
>>>>
>>>>> Signed-off-by: Hangyu Hua <hbh25y@gmail.com>
>>>>> ---
>>>>>   net/ax25/af_ax25.c | 4 +++-
>>>>>   1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/net/ax25/af_ax25.c b/net/ax25/af_ax25.c
>>>>> index cfca99e295b8..c5d62420a2a8 100644
>>>>> --- a/net/ax25/af_ax25.c
>>>>> +++ b/net/ax25/af_ax25.c
>>>>> @@ -1127,7 +1127,7 @@ static int __must_check ax25_connect(struct 
>>>>> socket *sock,
>>>>>       struct sockaddr *uaddr, int addr_len, int flags)
>>>>>   {
>>>>>       struct sock *sk = sock->sk;
>>>>> -    ax25_cb *ax25 = sk_to_ax25(sk), *ax25t;
>>>>> +    ax25_cb *ax25, *ax25t;
>>>>>       struct full_sockaddr_ax25 *fsa = (struct full_sockaddr_ax25 
>>>>> *)uaddr;
>>>>>       ax25_digi *digi = NULL;
>>>>>       int ct = 0, err = 0;
>>>>> @@ -1155,6 +1155,8 @@ static int __must_check ax25_connect(struct 
>>>>> socket *sock,
>>>>>       lock_sock(sk);
>>>>> +    ax25 = sk_to_ax25(sk);
>>>>> +
>>>>>       /* deal with restarts */
>>>>>       if (sock->state == SS_CONNECTING) {
>>>>>           switch (sk->sk_state) {