9p/xen : Fix use after free bug in xen_9pfs_front_remove due to race condition

maillist inclusion
category: bugfix
bugzilla: 188655, https://gitee.com/src-openeuler/kernel/issues/I6T36H
CVE: CVE-2023-1859

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/commit/?id=ea4f1009408efb4989a0f139b70fb338e7f687d0



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In xen_9pfs_front_probe, it calls xen_9pfs_front_alloc_dataring
to init priv->rings and bound &ring->work with p9_xen_response.

When it calls xen_9pfs_front_event_handler to handle IRQ requests,
it will finally call schedule_work to start the work.

When we call xen_9pfs_front_remove to remove the driver, there
may be a sequence as follows:

Fix it by finishing the work before cleanup in xen_9pfs_front_free.

Note that, this bug is found by static analysis, which might be
false positive.

CPU0                  CPU1

                     |p9_xen_response
xen_9pfs_front_remove|
  xen_9pfs_front_free|
kfree(priv)          |
//free priv          |
                     |p9_tag_lookup
                     |//use priv->client

Fixes: 71ebd719 ("xen/9pfs: connect to the backend")
Signed-off-by: Zheng Wang <zyytlz.wz@163.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Signed-off-by: Eric Van Hensbergen <ericvh@kernel.org>
Signed-off-by: Lu Wei <luwei32@huawei.com>
Reviewed-by: Yue Haibing <yuehaibing@huawei.com>
Reviewed-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>