[RFC PATCH net v2 2/2] net/smc: Resolve the race between SMC-R link access and clear

Wen Gu
2021-12-28T15:14:21Z

We encountered some crashes caused by the race between SMC-R
link access and link clear triggered by link group termination
in abnormal case, like port error.

Here are some of panic stacks we met:

1) Race between smc_llc_flow_initiate() and smcr_link_clear()

BUG: kernel NULL pointer dereference, address: 0000000000000000
Workqueue: smc_hs_wq smc_listen_work [smc]
RIP: 0010:smc_llc_flow_initiate+0x44/0x190 [smc]
Call Trace:
<TASK>
? __smc_buf_create+0x75a/0x950 [smc]
smcr_lgr_reg_rmbs+0x2a/0xbf [smc]
smc_listen_work+0xf72/0x1230 [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>

smc_listen_work() __smc_lgr_terminate()
---------------------------------------------------------------
| smc_lgr_free()
| |- smcr_link_clear()
| |- memset(lnk, 0)
smc_listen_rdma_reg() |
|- smcr_lgr_reg_rmbs() |
|- smc_llc_flow_initiate() |
|- access lnk->lgr (panic) |

2) Race between smc_wr_tx_dismiss_slots() and smcr_link_clear()

BUG: kernel NULL pointer dereference, address: 0000000000000000
RIP: 0010:_find_first_bit+0x8/0x50
Call Trace:
<TASK>
smc_wr_tx_dismiss_slots+0x34/0xc0 [smc]
? smc_cdc_tx_filter+0x10/0x10 [smc]
smc_conn_free+0xd8/0x100 [smc]
__smc_release+0xf1/0x140 [smc]
smc_release+0x89/0x1b0 [smc]
__sock_release+0x37/0xb0
sock_close+0x14/0x20
__fput+0xa9/0x260
task_work_run+0x6b/0xb0
do_exit+0x3ef/0xd40
do_group_exit+0x47/0xb0
__x64_sys_exit_group+0x14/0x20
do_syscall_64+0x34/0x90
entry_SYSCALL_64_after_hwframe+0x44/0xae
</TASK>

smc_conn_free() __smc_lgr_terminate()
----------------------------------------------------------------
| smc_lgr_free()
| |- smcr_link_clear()
| |- smc_wr_free_link_mem()
| |- lnk->wr_tx_mask = NULL;
smc_wr_tx_dismiss_slots() |
|- for_each_set_bit(link->wr_tx_mask) |
|- (panic) |

These crashes are caused by clearing SMC-R link resources
when someone is still accessing to them. So this patch tries
to fix it by introducing reference count of SMC-R links and
ensuring that the sensitive resources of links are not
cleared until reference count is zero.

The operation to the SMC-R link reference count can be concluded
as follows:

object [hold or initialized as 1] [put]
--------------------------------------------------------------------
links smcr_link_init() smcr_link_clear()
connections smcr_lgr_conn_assign_link() smc_conn_free()

Through this way, the clear of SMC-R links is later than the
free of all the smc connections above it, thus avoiding the
unsafe reference to SMC-R links.

Signed-off-by: Wen Gu <guwen@linux.alibaba.com>
---
net/smc/smc_core.c | 43 +++++++++++++++++++++++++++++++++++--------
net/smc/smc_core.h | 4 ++++
2 files changed, 39 insertions(+), 8 deletions(-)

diff --git a/net/smc/smc_core.c b/net/smc/smc_core.c
index d72eb13..a64d394 100644
--- a/net/smc/smc_core.c
+++ b/net/smc/smc_core.c
@@ -155,6 +155,7 @@ static int smcr_lgr_conn_assign_link(struct smc_connection *conn, bool first)
if (!conn->lnk)
return SMC_CLC_DECL_NOACTLINK;
atomic_inc(&conn->lnk->conn_cnt);
+ smcr_link_hold(conn->lnk); /* link_put in smc_conn_free() */
return 0;
}

@@ -746,6 +747,8 @@ int smcr_link_init(struct smc_link_group *lgr, struct smc_link *lnk,
}
get_device(&lnk->smcibdev->ibdev->dev);
atomic_inc(&lnk->smcibdev->lnk_cnt);
+ refcount_set(&lnk->refcnt, 1); /* link refcnt is set to 1 */
+ lnk->clearing = 0;
lnk->path_mtu = lnk->smcibdev->pattr[lnk->ibport - 1].active_mtu;
lnk->link_id = smcr_next_link_id(lgr);
lnk->lgr = lgr;
@@ -994,8 +997,12 @@ void smc_switch_link_and_count(struct smc_connection *conn,
struct smc_link *to_lnk)
{
atomic_dec(&conn->lnk->conn_cnt);
+ /* put old link, hold in smcr_lgr_conn_assign_link() */
+ smcr_link_put(conn->lnk);
conn->lnk = to_lnk;
atomic_inc(&conn->lnk->conn_cnt);
+ /* hold new link, put in smc_conn_free() */
+ smcr_link_hold(conn->lnk);
}

struct smc_link *smc_switch_conns(struct smc_link_group *lgr,
@@ -1126,9 +1133,9 @@ void smc_conn_free(struct smc_connection *conn)
/* 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.
+ * Judge these to ensure that lgr/link refcnt will be
+ * put only once if connection has been registered to
+ * a link group successfully.
*/
return;

@@ -1153,6 +1160,8 @@ void smc_conn_free(struct smc_connection *conn)
if (!lgr->conns_num)
smc_lgr_schedule_free_work(lgr);
lgr_put:
+ if (!lgr->is_smcd)
+ smcr_link_put(conn->lnk); /* link_hold in smcr_lgr_conn_assign_link() */
smc_lgr_put(lgr); /* lgr_hold in smc_lgr_register_conn() */
}

@@ -1209,13 +1218,23 @@ static void smcr_rtoken_clear_link(struct smc_link *lnk)
}
}

+static void __smcr_link_clear(struct smc_link *lnk)
+{
+ smc_wr_free_link_mem(lnk);
+ smc_lgr_put(lnk->lgr); /* lgr_hold in smcr_link_init() */
+ memset(lnk, 0, sizeof(struct smc_link));
+ lnk->state = SMC_LNK_UNUSED;
+}
+
/* must be called under lgr->llc_conf_mutex lock */
void smcr_link_clear(struct smc_link *lnk, bool log)
{
struct smc_ib_device *smcibdev;

- if (!lnk->lgr || lnk->state == SMC_LNK_UNUSED)
+ if (lnk->clearing || !lnk->lgr ||
+ lnk->state == SMC_LNK_UNUSED)
return;
+ lnk->clearing = 1;
lnk->peer_qpn = 0;
smc_llc_link_clear(lnk, log);
smcr_buf_unmap_lgr(lnk);
@@ -1224,15 +1243,23 @@ void smcr_link_clear(struct smc_link *lnk, bool log)
smc_wr_free_link(lnk);
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;
- memset(lnk, 0, sizeof(struct smc_link));
- lnk->state = SMC_LNK_UNUSED;
if (!atomic_dec_return(&smcibdev->lnk_cnt))
wake_up(&smcibdev->lnks_deleted);
+ smcr_link_put(lnk); /* theoretically last link_put */
+}
+
+void smcr_link_hold(struct smc_link *lnk)
+{
+ refcount_inc(&lnk->refcnt);
+}
+
+void smcr_link_put(struct smc_link *lnk)
+{
+ if (refcount_dec_and_test(&lnk->refcnt))
+ __smcr_link_clear(lnk);
}

static void smcr_buf_free(struct smc_link_group *lgr, bool is_rmb,
diff --git a/net/smc/smc_core.h b/net/smc/smc_core.h
index 51203b1..e73217f 100644
--- a/net/smc/smc_core.h
+++ b/net/smc/smc_core.h
@@ -137,6 +137,8 @@ struct smc_link {
u8 peer_link_uid[SMC_LGR_ID_SIZE]; /* peer uid */
u8 link_idx; /* index in lgr link array */
u8 link_is_asym; /* is link asymmetric? */
+ u8 clearing : 1; /* link is being cleared */
+ refcount_t refcnt; /* link reference count */
struct smc_link_group *lgr; /* parent link group */
struct work_struct link_down_wrk; /* wrk to bring link down */
char ibname[IB_DEVICE_NAME_MAX]; /* ib device name */
@@ -504,6 +506,8 @@ void smc_rtoken_set2(struct smc_link_group *lgr, int rtok_idx, int link_id,
int smcr_link_init(struct smc_link_group *lgr, struct smc_link *lnk,
u8 link_idx, struct smc_init_info *ini);
void smcr_link_clear(struct smc_link *lnk, bool log);
+void smcr_link_hold(struct smc_link *lnk);
+void smcr_link_put(struct smc_link *lnk);
void smc_switch_link_and_count(struct smc_connection *conn,
struct smc_link *to_lnk);
int smcr_buf_map_lgr(struct smc_link *lnk);
--
1.8.3.1