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

[Wenjia Zhang <wenjia@xxxxxxxxxxxxx>]

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?

>   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?

> Fixes: 8cf3f3e42374 ("net/smc: use helper smc_conn_abort() in listen processing")
> Co-developed-by: Guangguan Wang <guangguan.wang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guangguan Wang <guangguan.wang@xxxxxxxxxxxxxxxxx>
> Co-developed-by: Kai <KaiShen@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Kai <KaiShen@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Wen Gu <guwen@xxxxxxxxxxxxxxxxx>
> ---
>   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?

Thanks,
Wenjia