netdev - Re: [PATCH net] ax25: use after free in ax25

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Message-ID: <80007b3e-eba8-1fbe-302d-4398830843dd@gmail.com>
Date:   Wed, 12 Jan 2022 19:11:05 +0800
From:   Hangyu Hua <hbh25y@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>, jreuter@...na.de,
        ralf@...ux-mips.org, davem@...emloft.net, kuba@...nel.org
Cc:     linux-hams@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] ax25: use after free in ax25_connect

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@...il.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) {