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Subject[RFC PATCH net 1/2] net/smc: Resolve the race between link group access and termination
Date
We encountered some crashes caused by the race between the access
and the termination of link groups.

Here are some of panic stacks we met:

1) Race between smc_clc_wait_msg() and __smc_lgr_terminate()

BUG: kernel NULL pointer dereference, address: 00000000000002f0
Workqueue: smc_hs_wq smc_listen_work [smc]
RIP: 0010:smc_clc_wait_msg+0x3eb/0x5c0 [smc]
Call Trace:
<TASK>
? smc_clc_send_accept+0x45/0xa0 [smc]
? smc_clc_send_accept+0x45/0xa0 [smc]
smc_listen_work+0x783/0x1220 [smc]
? finish_task_switch+0xc4/0x2e0
? process_one_work+0x1ad/0x3c0
process_one_work+0x1ad/0x3c0
worker_thread+0x4c/0x390
? rescuer_thread+0x320/0x320
kthread+0x149/0x190
? set_kthread_struct+0x40/0x40
ret_from_fork+0x1f/0x30
</TASK>

smc_listen_work() abnormal case like port error
---------------------------------------------------------------
| __smc_lgr_terminate()
| |- smc_conn_kill()
| |- smc_lgr_unregister_conn()
| |- set conn->lgr = NULL
smc_clc_wait_msg() |
|- access conn->lgr (panic) |

2) Race between smc_setsockopt() and __smc_lgr_terminate()

BUG: kernel NULL pointer dereference, address: 00000000000002e8
RIP: 0010:smc_setsockopt+0x17a/0x280 [smc]
Call Trace:
<TASK>
__sys_setsockopt+0xfc/0x190
__x64_sys_setsockopt+0x20/0x30
do_syscall_64+0x34/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xae
</TASK>

smc_setsockopt() abnormal case like port error
--------------------------------------------------------------
| __smc_lgr_terminate()
| |- smc_conn_kill()
| |- smc_lgr_unregister_conn()
| |- set conn->lgr = NULL
mod_delayed_work() |
|- access conn->lgr (panic) |

There are some other panic points and they are caused by the
simmilar reason as described above, which is accessing link
group after termination, thus getting a NULL pointer or invalid
resource.

Currently, there seems to be no synchronization between the
link group access and a sudden termination of it. This patch
tries to fix this by introducing reference count of link group
and not freeing link group until reference count is zero.

Link group might be referred to by link or smc connection. So
the operation to the link group reference count can be concluded
as follows:

object [hold or initialized as 1] [put]
--------------------------------------------------------------------
link group smc_lgr_create() smc_lgr_free()
connections smc_lgr_register_conn() smc_conn_free()
links smcr_link_init() smcr_link_clear()

Througth this way, we extend the life cycle of link group and
ensure it is longer than the life cycle of connections and links
above it, so that avoid invalid access to link group after its
termination.

Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
---
net/smc/smc.h | 1 +
net/smc/smc_core.c | 45 ++++++++++++++++++++++++++++++++++++++++-----
net/smc/smc_core.h | 3 +++
3 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/net/smc/smc.h b/net/smc/smc.h
index 1a4fc1c..3d0b8e3 100644
--- a/net/smc/smc.h
+++ b/net/smc/smc.h
@@ -221,6 +221,7 @@ struct smc_connection {
*/
u64 peer_token; /* SMC-D token of peer */
u8 killed : 1; /* abnormal termination */
+ u8 freed : 1; /* normal termiation */
u8 out_of_sync : 1; /* out of sync with peer */
};

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index 1f40b8e..d72eb13 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -184,6 +184,7 @@ static int smc_lgr_register_conn(struct smc_connection *conn, bool first)
conn->alert_token_local = 0;
}
smc_lgr_add_alert_token(conn);
+ smc_lgr_hold(conn->lgr); /* lgr_put in smc_conn_free() */
conn->lgr->conns_num++;
return 0;
}
@@ -216,7 +217,6 @@ static void smc_lgr_unregister_conn(struct smc_connection *conn)
__smc_lgr_unregister_conn(conn);
}
write_unlock_bh(&lgr->conns_lock);
- conn->lgr = NULL;
}

int smc_nl_get_sys_info(struct sk_buff *skb, struct netlink_callback *cb)
@@ -749,6 +749,7 @@ int smcr_link_init(struct smc_link_group *lgr, struct smc_link *lnk,
lnk->path_mtu = lnk->smcibdev->pattr[lnk->ibport - 1].active_mtu;
lnk->link_id = smcr_next_link_id(lgr);
lnk->lgr = lgr;
+ smc_lgr_hold(lgr); /* lgr_put in smcr_link_clear() */
lnk->link_idx = link_idx;
smc_ibdev_cnt_inc(lnk);
smcr_copy_dev_info_to_link(lnk);
@@ -841,6 +842,7 @@ static int smc_lgr_create(struct smc_sock *smc, struct smc_init_info *ini)
lgr->terminating = 0;
lgr->freeing = 0;
lgr->vlan_id = ini->vlan_id;
+ refcount_set(&lgr->refcnt, 1); /* set lgr refcnt to 1 */
mutex_init(&lgr->sndbufs_lock);
mutex_init(&lgr->rmbs_lock);
rwlock_init(&lgr->conns_lock);
@@ -1120,8 +1122,20 @@ void smc_conn_free(struct smc_connection *conn)
{
struct smc_link_group *lgr = conn->lgr;

- if (!lgr)
+ if (!lgr || conn->freed)
+ /* smc connection wasn't registered to a link group
+ * or has already been freed before.
+ *
+ * Judge these to ensure that lgr refcnt will be put
+ * only once if connection has been registered to a
+ * link group successfully.
+ */
return;
+
+ conn->freed = 1;
+ if (conn->killed)
+ goto lgr_put;
+
if (lgr->is_smcd) {
if (!list_empty(&lgr->list))
smc_ism_unset_conn(conn);
@@ -1138,6 +1152,8 @@ void smc_conn_free(struct smc_connection *conn)

if (!lgr->conns_num)
smc_lgr_schedule_free_work(lgr);
+lgr_put:
+ smc_lgr_put(lgr); /* lgr_hold in smc_lgr_register_conn() */
}

/* unregister a link from a buf_desc */
@@ -1209,6 +1225,7 @@ void smcr_link_clear(struct smc_link *lnk, bool log)
smc_ib_destroy_queue_pair(lnk);
smc_ib_dealloc_protection_domain(lnk);
smc_wr_free_link_mem(lnk);
+ smc_lgr_put(lnk->lgr); /* lgr_hold in smcr_link_init() */
smc_ibdev_cnt_dec(lnk);
put_device(&lnk->smcibdev->ibdev->dev);
smcibdev = lnk->smcibdev;
@@ -1283,6 +1300,15 @@ static void smc_lgr_free_bufs(struct smc_link_group *lgr)
__smc_lgr_free_bufs(lgr, true);
}

+/* won't be freed until no one accesses to lgr anymore */
+static void __smc_lgr_free(struct smc_link_group *lgr)
+{
+ smc_lgr_free_bufs(lgr);
+ if (!lgr->is_smcd)
+ smc_wr_free_lgr_mem(lgr);
+ kfree(lgr);
+}
+
/* remove a link group */
static void smc_lgr_free(struct smc_link_group *lgr)
{
@@ -1298,7 +1324,6 @@ static void smc_lgr_free(struct smc_link_group *lgr)
smc_llc_lgr_clear(lgr);
}

- smc_lgr_free_bufs(lgr);
destroy_workqueue(lgr->tx_wq);
if (lgr->is_smcd) {
smc_ism_put_vlan(lgr->smcd, lgr->vlan_id);
@@ -1306,11 +1331,21 @@ static void smc_lgr_free(struct smc_link_group *lgr)
if (!atomic_dec_return(&lgr->smcd->lgr_cnt))
wake_up(&lgr->smcd->lgrs_deleted);
} else {
- smc_wr_free_lgr_mem(lgr);
if (!atomic_dec_return(&lgr_cnt))
wake_up(&lgrs_deleted);
}
- kfree(lgr);
+ smc_lgr_put(lgr); /* theoretically last lgr_put */
+}
+
+void smc_lgr_hold(struct smc_link_group *lgr)
+{
+ refcount_inc(&lgr->refcnt);
+}
+
+void smc_lgr_put(struct smc_link_group *lgr)
+{
+ if (refcount_dec_and_test(&lgr->refcnt))
+ __smc_lgr_free(lgr);
}

static void smc_sk_wake_ups(struct smc_sock *smc)
diff --git a/net/smc/smc_core.h b/net/smc/smc_core.h
index d63b082..51203b1 100644
--- a/net/smc/smc_core.h
+++ b/net/smc/smc_core.h
@@ -249,6 +249,7 @@ struct smc_link_group {
u8 terminating : 1;/* lgr is terminating */
u8 freeing : 1; /* lgr is being freed */

+ refcount_t refcnt; /* lgr reference count */
bool is_smcd; /* SMC-R or SMC-D */
u8 smc_version;
u8 negotiated_eid[SMC_MAX_EID_LEN];
@@ -470,6 +471,8 @@ static inline void smc_set_pci_values(struct pci_dev *pci_dev,

void smc_lgr_cleanup_early(struct smc_connection *conn);
void smc_lgr_terminate_sched(struct smc_link_group *lgr);
+void smc_lgr_hold(struct smc_link_group *lgr);
+void smc_lgr_put(struct smc_link_group *lgr);
void smcr_port_add(struct smc_ib_device *smcibdev, u8 ibport);
void smcr_port_err(struct smc_ib_device *smcibdev, u8 ibport);
void smc_smcd_terminate(struct smcd_dev *dev, u64 peer_gid,
--
1.8.3.1
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