You may not need this plugin since serverless/serverless#6127 was merged.
Resolve serverless invalid principal issue when deploying in China region.
$ npm i serverless-plugin-aws-cn-principal -D
/* serverless.yml */
# ...
plugins:
- serverless-plugin-aws-cn-principal
provider:
name: aws
region: cn-north-1
endpointType: regional
# ...
$ serverless deploy -v