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

[Wen Gu <guwen@linux.alibaba.com>]

On 2024/11/23 00:03, Alexandra Winter 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.
>>
>>   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.
>>
>> 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;
>>   
> 
> I wonder if this can deadlock, when you take lock_sock so far down in the callchain.
> example:
>   smc_connect will first take lock_sock(sk) and then mutex_lock(&smc_server_lgr_pending);  (e.g. in smc_connect_ism())
> wheras
> smc_listen_work() will take mutex_lock(&smc_server_lgr_pending); and then lock_sock(sk) (in your __smc_conn_abort(,,false))
> 
> I am not sure whether this can be called on the same socket, but it looks suspicious to me.
> 

IMHO this two paths can not occur on the same sk.

> 
> All callers of smc_conn_abort() without socklock seem to originate from smc_listen_work().
> That makes me think whether smc_listen_work() should do lock_sock(sk) on a higher level.
> 

Yes, I also think about this question, I guess it is because the new smc sock will be
accepted by userspace only after smc_listen_work() is completed. Before that, no userspace
operation occurs synchronously with it, so it is not protected by sock lock. But I am not
sure if there are other reasons, so I did not aggressively protect the entire smc_listen_work
with sock lock, but chose a conservative approach.

> Do you have an example which function could collide with smc_listen_work()?
> i.e. have you found a way to reproduce this?
> 

We discovered this during our fault injection testing where the rdma driver was rmmod/insmod
sporadically during the nginx/wrk 1K connections test.

e.g.

    __smc_lgr_terminate            | smc_listen_decline
    (caused by rmmod mlx5_ib)      | (caused by e.g. reg mr fail)
    --------------------------------------------------------------
    lock_sock                      |
    smc_conn_kill                  | smc_conn_abort
     \- smc_conn_free              |  \- smc_conn_free
    release_sock                   |

> 
> Are you sure that all callers of smc_conn_free(), that are not smc_conn_abort(), do set the socklock?
> It seems to me that the path of smc_conn_kill() is not covered by your solution.
> 

smc_conn_free is called in these places:

1. __smc_release (protected by sock lock)
2. smc_conn_abort (partially protected by sock lock)
3. smc_close_active_abort - smc_release(protected by sock lock)
                           - smc_conn_kill - __smc_lgr_terminate/smc_conn_abort_work(protected by sock lock)
4. smc_close_passive_work (protected by sock lock)

So only smc_conn_abort->smc_conn_free is not well protected by sock lock.

> 
> Please excuse, that I am not deeply familiar with this code.
> I'm just trying to ask helpful questions.

Thanks! :)