Re: [PATCH net] ax25: use after free in ax25_connect

[Eric Dumazet <eric.dumazet@gmail.com>]

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) {