[PATCH] net: fix a data race when get vlan device

[zhudi <zhudi21@huawei.com>]

From: Di Zhu <zhudi21@huawei.com>

We encountered a crash: in the packet receiving process, we got an
illegal VLAN device address, but the VLAN device address saved in vmcore
is correct. After checking the code, we found a possible data
competition:
CPU 0:                             CPU 1:
    (RCU read lock)                  (RTNL lock)
    vlan_do_receive()		       register_vlan_dev()
      vlan_find_dev()

        ->__vlan_group_get_device()	 ->vlan_group_prealloc_vid()

In vlan_group_prealloc_vid(), We need to make sure that kzalloc is
executed before assigning a value to vlan devices array, otherwise we
may get a wrong address from the hardware cache on another cpu.

So fix it by adding memory barrier instruction to ensure the order
of memory operations.

Signed-off-by: Di Zhu <zhudi21@huawei.com>
---
 net/8021q/vlan.c | 2 ++
 net/8021q/vlan.h | 3 +++
 2 files changed, 5 insertions(+)

diff --git a/net/8021q/vlan.c b/net/8021q/vlan.c
index 8b644113715e..4f541e05cd3f 100644
--- a/net/8021q/vlan.c
+++ b/net/8021q/vlan.c
@@ -71,6 +71,8 @@ static int vlan_group_prealloc_vid(struct vlan_group *vg,
 	if (array == NULL)
 		return -ENOBUFS;
 
+	smp_wmb();
+
 	vg->vlan_devices_arrays[pidx][vidx] = array;
 	return 0;
 }
diff --git a/net/8021q/vlan.h b/net/8021q/vlan.h
index 953405362795..7408fda084d3 100644
--- a/net/8021q/vlan.h
+++ b/net/8021q/vlan.h
@@ -57,6 +57,9 @@ static inline struct net_device *__vlan_group_get_device(struct vlan_group *vg,
 
 	array = vg->vlan_devices_arrays[pidx]
 				       [vlan_id / VLAN_GROUP_ARRAY_PART_LEN];
+
+	smp_rmb();
+
 	return array ? array[vlan_id % VLAN_GROUP_ARRAY_PART_LEN] : NULL;
 }
 
-- 
2.23.0