Re: [PATCH net 2/2] net/smc: fix LGR and link use-after-free issue

Wenjia Zhang
2024-11-26T12:19:45Z

On 26.11.24 13:12, Wenjia Zhang wrote:
>
>
> On 25.11.24 07:46, Wen Gu wrote:
>>
>>
>> On 2024/11/22 23:56, Wenjia Zhang wrote:
>>>
>>>
>>> On 22.11.24 08:16, Wen Gu wrote:
>>>> We encountered a LGR/link use-after-free issue, which manifested as
>>>> the LGR/link refcnt reaching 0 early and entering the clear process,
>>>> making resource access unsafe.
>>>>
>>>
>>> How did you make sure that the refcount mentioned in the warning are
>>> the LGR/link refcnt, not &sk->sk_refcnt?
>>>
>> Because according to the panic stack, the UAF is found in
>> smcr_link_put(),
>> and it is also found that the link has been cleared at that time (lnk has
>> been memset to all zero by __smcr_link_clear()).
>>
> ok, I think you're right, I was distracted by the the sock_put() in
> function __smc_lgr_terminate()
>
>>>>   refcount_t: addition on 0; use-after-free.
>>>>   WARNING: CPU: 14 PID: 107447 at lib/refcount.c:25
>>>> refcount_warn_saturate+0x9c/0x140
>>>>   Workqueue: events smc_lgr_terminate_work [smc]
>>>>   Call trace:
>>>>    refcount_warn_saturate+0x9c/0x140
>>>>    __smc_lgr_terminate.part.45+0x2a8/0x370 [smc]
>>>>    smc_lgr_terminate_work+0x28/0x30 [smc]
>>>>    process_one_work+0x1b8/0x420
>>>>    worker_thread+0x158/0x510
>>>>    kthread+0x114/0x118
>>>>
>>>> or
>>>>
>>>>   refcount_t: underflow; use-after-free.
>>>>   WARNING: CPU: 6 PID: 93140 at lib/refcount.c:28
>>>> refcount_warn_saturate+0xf0/0x140
>>>>   Workqueue: smc_hs_wq smc_listen_work [smc]
>>>>   Call trace:
>>>>    refcount_warn_saturate+0xf0/0x140
>>>>    smcr_link_put+0x1cc/0x1d8 [smc]
>>>>    smc_conn_free+0x110/0x1b0 [smc]
>>>>    smc_conn_abort+0x50/0x60 [smc]
>>>>    smc_listen_find_device+0x75c/0x790 [smc]
>>>>    smc_listen_work+0x368/0x8a0 [smc]
>>>>    process_one_work+0x1b8/0x420
>>>>    worker_thread+0x158/0x510
>>>>    kthread+0x114/0x118
>>>>
>>>> It is caused by repeated release of LGR/link refcnt. One suspect is
>>>> that
>>>> smc_conn_free() is called repeatedly because some smc_conn_free()
>>>> are not
>>>> protected by sock lock.
>>>>
>>>> Calls under socklock        | Calls not under socklock
>>>> -------------------------------------------------------
>>>> lock_sock(sk)               | smc_conn_abort
>>>> smc_conn_free               | \- smc_conn_free
>>>> \- smcr_link_put            |    \- smcr_link_put (duplicated)
>>>> release_sock(sk)
>>>>
>>>> So make sure smc_conn_free() is called under the sock lock.
>>>>
>>>
>>> If I understand correctly, the fix could only solve a part of the
>>> problem, i.e. what the second call trace reported, right?
>>
>> I think that these panic stacks (there are some other variations that
>> I haven't posted)
>> have the same root cause, that is the link/lgr refcnt reaches 0 early
>> in the race situation,
>> making access to link/lgr related resources no longer safe.
>>
>> The link/lgr refcnt was introduced by [1] & [2], the link refcnt is
>> operated by link
>> itself and connections registered to it, and the lgr refcnt is
>> operated by lgr itself,
>> links belong to it and connections registered to it. Through code
>> analysis, the most
>> likely suspect is that smc_conn_free() duplicate put link/lgr refcnt
>> because some
>> smc_conn_free() calls by smc_conn_abort() are not under the protection
>> of sock lock,
>> so if they are called at the same time, there may be a race condition.
>>
>> for example:
>>
>>     __smc_lgr_terminate            | smc_listen_decline
>>     --------------------------------------------------------------
>>     lock_sock                      |
>>     smc_conn_kill                  | smc_conn_abort
>>      \- smc_conn_free              |  \- smc_conn_free
>>     release_sock                   |
>>
>> [1] 61f434b0280e ("net/smc: Resolve the race between link group access
>> and termination")
>> [2] 20c9398d3309 ("net/smc: Resolve the race between SMC-R link access
>> and clear")
>>
> I see, thx!
>>>
>>>> Fixes: 8cf3f3e42374 ("net/smc: use helper smc_conn_abort() in listen
>>>> processing")
>>>> Co-developed-by: Guangguan Wang <guangguan.wang@linux.alibaba.com>
>>>> Signed-off-by: Guangguan Wang <guangguan.wang@linux.alibaba.com>
>>>> Co-developed-by: Kai <KaiShen@linux.alibaba.com>
>>>> Signed-off-by: Kai <KaiShen@linux.alibaba.com>
>>>> Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
>>>> ---
>>>>   net/smc/af_smc.c | 25 +++++++++++++++++++++----
>>>>   1 file changed, 21 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
>>>> index ed6d4d520bc7..e0a7a0151b11 100644
>>>> --- a/net/smc/af_smc.c
>>>> +++ b/net/smc/af_smc.c
>>>> @@ -973,7 +973,8 @@ static int smc_connect_decline_fallback(struct
>>>> smc_sock *smc, int reason_code,
>>>>       return smc_connect_fallback(smc, reason_code);
>>>>   }
>>>> -static void smc_conn_abort(struct smc_sock *smc, int local_first)
>>>> +static void __smc_conn_abort(struct smc_sock *smc, int local_first,
>>>> +                 bool locked)
>>>>   {
>>>>       struct smc_connection *conn = &smc->conn;
>>>>       struct smc_link_group *lgr = conn->lgr;
>>>> @@ -982,11 +983,27 @@ static void smc_conn_abort(struct smc_sock
>>>> *smc, int local_first)
>>>>       if (smc_conn_lgr_valid(conn))
>>>>           lgr_valid = true;
>>>> -    smc_conn_free(conn);
>>>> +    if (!locked) {
>>>> +        lock_sock(&smc->sk);
>>>> +        smc_conn_free(conn);
>>>> +        release_sock(&smc->sk);
>>>> +    } else {
>>>> +        smc_conn_free(conn);
>>>> +    }
>>>>       if (local_first && lgr_valid)
>>>>           smc_lgr_cleanup_early(lgr);
>>>>   }
>>>> +static void smc_conn_abort(struct smc_sock *smc, int local_first)
>>>> +{
>>>> +    __smc_conn_abort(smc, local_first, false);
>>>> +}
>>>> +
>>>> +static void smc_conn_abort_locked(struct smc_sock *smc, int
>>>> local_first)
>>>> +{
>>>> +    __smc_conn_abort(smc, local_first, true);
>>>> +}
>>>> +
>>>>   /* check if there is a rdma device available for this connection. */
>>>>   /* called for connect and listen */
>>>>   static int smc_find_rdma_device(struct smc_sock *smc, struct
>>>> smc_init_info *ini)
>>>> @@ -1352,7 +1369,7 @@ static int smc_connect_rdma(struct smc_sock *smc,
>>>>       return 0;
>>>>   connect_abort:
>>>> -    smc_conn_abort(smc, ini->first_contact_local);
>>>> +    smc_conn_abort_locked(smc, ini->first_contact_local);
>>>>       mutex_unlock(&smc_client_lgr_pending);
>>>>       smc->connect_nonblock = 0;
>>>> @@ -1454,7 +1471,7 @@ static int smc_connect_ism(struct smc_sock *smc,
>>>>       return 0;
>>>>   connect_abort:
>>>> -    smc_conn_abort(smc, ini->first_contact_local);
>>>> +    smc_conn_abort_locked(smc, ini->first_contact_local);
>>>>       mutex_unlock(&smc_server_lgr_pending);
>>>>       smc->connect_nonblock = 0;
>>>
>>> Why is smc_conn_abort_locked() only necessary for the
>>> smc_connect_work, not for the smc_listen_work?
>>>
>>
>> Before this patch, the smc_conn_abort()->smc_conn_free() calls are not
>> protected by sock lock except in smc_conn_{rdma|ism}. So I add sock lock
> Do you mean here that the smc_conn_abort()->smc_conn_free() calls are
> protected in smc_listen_{rdma|ism}, right?
> If it is, could you please point me out where they(the protection) are?

sorry, I see it, pls forget this question
>> protection inside the __smc_conn_abort() and introduce
>> smc_conn_abort_locked()
>> (which means sock lock has been taken) for smc_conn_{rdma|ism}.
>>
>
> Thanks,
> Wenjia
>
>