Reusing the msg_id after a maliciously completed reopen request may cause
a read request to remain unprocessed and result in a hung, as shown below:

       t1       |      t2       |      t3
-------------------------------------------------
cachefiles_ondemand_select_req
 cachefiles_ondemand_object_is_close(A)
 cachefiles_ondemand_set_object_reopening(A)
 queue_work(fscache_object_wq, &info->work)
                ondemand_object_worker
                 cachefiles_ondemand_init_object(A)
                  cachefiles_ondemand_send_req(OPEN)
                    // get msg_id 6
                    wait_for_completion(&req_A->done)
cachefiles_ondemand_daemon_read
 // read msg_id 6 req_A
 cachefiles_ondemand_get_fd
 copy_to_user
                                // Malicious completion msg_id 6
                                copen 6,-1
                                cachefiles_ondemand_copen
                                 complete(&req_A->done)
                                 // will not set the object to close
                                 // because ondemand_id && fd is valid.

                // ondemand_object_worker() is done
                // but the object is still reopening.

                                // new open req_B
                                cachefiles_ondemand_init_object(B)
                                 cachefiles_ondemand_send_req(OPEN)
                                 // reuse msg_id 6
process_open_req
 copen 6,A.size
 // The expected failed copen was executed successfully

Expect copen to fail, and when it does, it closes fd, which sets the
object to close, and then close triggers reopen again. However, due to
msg_id reuse resulting in a successful copen, the anonymous fd is not
closed until the daemon exits. Therefore read requests waiting for reopen
to complete may trigger hung task.

To avoid this issue, allocate the msg_id cyclically to avoid reusing the
msg_id for a very short duration of time.

Fixes: c8383054 ("cachefiles: notify the user daemon when looking up cookie")
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240628062930.2467993-9-libaokun@huaweicloud.comAcked-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Jia Zhu <zhujia.zj@bytedance.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>