Subject: Re: [PATCH net v3] net/smc: Reset conn->lgr when link group registration fails

From: "dust.li" <[email-masked]>
Date: Wed, 5 Jan 2022 15:54:08 +0800

On Wed, Jan 05, 2022 at 02:28:53PM +0800, Wen Gu wrote:
>SMC connections might fail to be registered to a link group due to
>things like unable to find a link to assign to in its creation. As
>a result, connection creation will return a failure and most
>resources related to the connection won't be applied or initialized,
>such as conn->abort_work or conn->lnk.
>
>If smc_conn_free() is invoked later, it will try to access the
>resources related to the connection, which wasn't initialized, thus
>causing a panic.
>
>Here is an example, a SMC-R connection failed to be registered
>to a link group and conn->lnk is NULL. The following crash will
>happen if smc_conn_free() tries to access conn->lnk in
>smc_cdc_tx_dismiss_slots().
>
> BUG: kernel NULL pointer dereference, address: 0000000000000168
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 4 PID: 68 Comm: kworker/4:1 Kdump: loaded Tainted: G E     5.16.0-rc5+ #52
> Workqueue: smc_hs_wq smc_listen_work [smc]
> RIP: 0010:smc_wr_tx_dismiss_slots+0x1e/0xc0 [smc]
> Call Trace:
>  <TASK>
>  smc_conn_free+0xd8/0x100 [smc]
>  smc_lgr_cleanup_early+0x15/0x90 [smc]
>  smc_listen_work+0x302/0x1230 [smc]
>  ? process_one_work+0x25c/0x600
>  process_one_work+0x25c/0x600
>  worker_thread+0x4f/0x3a0
>  ? process_one_work+0x600/0x600
>  kthread+0x15d/0x1a0
>  ? set_kthread_struct+0x40/0x40
>  ret_from_fork+0x1f/0x30
>  </TASK>
>
>This patch tries to fix this by resetting conn->lgr to NULL if an
>abnormal exit occurs in smc_lgr_register_conn(), thus avoiding the
>crash caused by accessing the uninitialized resources in smc_conn_free().
>And the new created link group will be terminated if smc connections
>can't be registered to it.
>
>Fixes: 56bc3b2094b4 ("net/smc: assign link to a new connection")
>Signed-off-by: Wen Gu <[email-masked]>
>---
>v1->v2:
>- Reset conn->lgr to NULL in smc_lgr_register_conn().
>- Only free new created link group.
>v2->v3:
>- Using __smc_lgr_terminate() instead of smc_lgr_schedule_free_work()
>  for an immediate free.
>---
> net/smc/smc_core.c | 12 ++++++++++--
> 1 file changed, 10 insertions(+), 2 deletions(-)
>
>diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
>index 412bc85..0201f99 100644
>--- a/net/smc/smc_core.c
>+++ b/net/smc/smc_core.c
>@@ -171,8 +171,10 @@ static int smc_lgr_register_conn(struct smc_connection *conn, bool first)
> 
> 	if (!conn->lgr->is_smcd) {
> 		rc = smcr_lgr_conn_assign_link(conn, first);
>-		if (rc)
>+		if (rc) {
>+			conn->lgr = NULL;
> 			return rc;
>+		}
> 	}
> 	/* find a new alert_token_local value not yet used by some connection
> 	 * in this link group
>@@ -1835,8 +1837,14 @@ int smc_conn_create(struct smc_sock *smc, struct smc_init_info *ini)
> 		write_lock_bh(&lgr->conns_lock);
> 		rc = smc_lgr_register_conn(conn, true);
> 		write_unlock_bh(&lgr->conns_lock);
>-		if (rc)
>+		if (rc) {
>+			spin_lock_bh(lgr_lock);
>+			if (!list_empty(&lgr->list))
>+				list_del_init(&lgr->list);
>+			spin_unlock_bh(lgr_lock);
>+			__smc_lgr_terminate(lgr, true);

What about adding a smc_lgr_terminate() wrapper and put list_del_init()
and __smc_lgr_terminate() into it ?

> 			goto out;
>+		}
> 	}
> 	conn->local_tx_ctrl.common.type = SMC_CDC_MSG_TYPE;
> 	conn->local_tx_ctrl.len = SMC_WR_TX_SIZE;
>-- 
>1.8.3.1