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Message-ID: <ff65d70b-b6e1-3b35-8bd0-92f6f022cd5d@gmail.com>
Date: Fri, 14 Jan 2022 14:54:31 +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
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@...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) {
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