一、前言

背景：一个S3 bucket，存储用户的文件，每个用户只允许上传、下载自己目录下的文件。

如何让Policy更灵活、更动态，可以让获取到的权限凭证可以匹配到单个终端用户的S3文件目录下。

本节主要介绍，以编程方式调用 AWS Security Token Service (AWS STS) 的API，获取访问AWS资源的临时安全凭证，并将这些凭证下发给终端用户，用于后续终端用户发起访问AWS资源请求时进行身份验证。

二、整体图

1. 终端用户的客户端发出访问请求到服务器端。
2. 服务器端调用STS API，以获取临时安全凭证。
3. STS服务返回临时安全凭证。
4. Server将获取到的临时安全凭证下发给客户端。
5. 客户端使用临时安全凭证来访问AWS资源(本文中指S3 bucket)。

三、程序

package main

import (
    "context"
    "errors"
    "fmt"
    "github.com/aws/aws-sdk-go-v2/aws"
    "github.com/aws/aws-sdk-go-v2/config"
    "github.com/aws/aws-sdk-go-v2/credentials"
    "github.com/aws/aws-sdk-go-v2/service/sts"
    "strconv"
    "strings"
    "time"
)

func main() {
    var bucketName = "{bucket名称}"
    var authPaths = []string{"uid"} //uid可替换用户真实uid对应的目录，可支持多层级目录
    var expire int32 = 3600 //STS token有效期
    cfg := &StoreClientConf{
        RoleArn:         "{roleArn}",
        Region:          "{bucket region}",
        AccessKeyID:     "{bucket ak}",
        AccessKeySecret: "{bucket sk}",
    }
    client := NewAwsClient(cfg)
    stsInfo, err := client.GetStsCredentials(context.Background(), bucketName, authPaths, expire)
    if err != nil {
        fmt.Println("client.GetStsCredentials err: " + err.Error())
        return
    }
    fmt.Println("sts ak: " + stsInfo.AccessKeyId)
    fmt.Println("sts sk: " + stsInfo.AccessSecret)
    fmt.Println("sts token: " + stsInfo.SecurityToken)
}

type AwsClient struct {
    roleArn         string
    region          string
    accessKeyID     string
    accessKeySecret string
}

type StoreClientConf struct {
    RoleArn         string
    Region          string
    AccessKeyID     string
    AccessKeySecret string
}

type StsCredentials struct {
    AccessKeyId   string
    AccessSecret  string
    SecurityToken string
    ExpireTime    int64
}

func NewAwsClient(cfg *StoreClientConf) *AwsClient {
    return &AwsClient{
        roleArn:         cfg.RoleArn,
        region:          cfg.Region,
        accessKeyID:     cfg.AccessKeyID,
        accessKeySecret: cfg.AccessKeySecret,
    }
}

func (s *AwsClient) loadConfig(ctx context.Context) (aws.Config, error) {
    cfg, err := config.LoadDefaultConfig(ctx,
        config.WithRegion(s.region),
        config.WithCredentialsProvider(credentials.StaticCredentialsProvider{
            Value: aws.Credentials{
                AccessKeyID: s.accessKeyID, SecretAccessKey: s.accessKeySecret, SessionToken: "",
                Source: "",
            },
        }),
    )
    if err != nil {
        fmt.Println("awsClient LoadDefaultConfig err:" + err.Error())
        return aws.Config{}, errors.New("awsClient LoadDefaultConfig err")
    }

    return cfg, nil
}

// 定义自己想要的policy
func (s *AwsClient) authPolicy(ctx context.Context, bucket string, authPaths []string) string {
    var resource []string
    for _, v := range authPaths {
        path := strings.TrimRight(v, "/") //去除最后一个/
        resource = append(resource, `"arn:aws:s3:::` + bucket+ `/` + path + `/*"`)
    }
    policy := `{
    "Version": "2012-10-17",
    "Statement": [
        {
            "Action": [
                "s3:GetObject",
                "s3:GetObjectAttributes",
                "s3:GetObjectTagging",
                "s3:PutObject",
                "s3:PutObjectTagging",
                "s3:UploadPart"
            ],
            "Effect": "Allow",
            "Resource": [` + strings.Join(resource, ",") + `]
        }
    ]
}`
    return policy
}

func (s *AwsClient) GetStsCredentials(ctx context.Context, bucket string, authPaths []string, expired int32) (*StsCredentials, error) {
    // 1.拼装授权策略
    policy := s.authPolicy(ctx, bucket, authPaths)

    // 2.初始化client
    cfg, err := s.loadConfig(ctx)
    if err != nil {
        return nil, err
    }
    client := sts.NewFromConfig(cfg)

    // 3.调用s3接口，获取sts token
    roleSessionName := "s3bucket" + strconv.FormatInt(time.Now().Unix(), 10) //需要按用户的维度去修改
    input := &sts.AssumeRoleInput{
        RoleArn:         &s.roleArn,
        RoleSessionName: &roleSessionName,
        DurationSeconds: &expired,
        Policy: aws.String(policy),
    }
    resp, err := client.AssumeRole(ctx, input)
    if err != nil {
        fmt.Println("GetStsCredentials client.AssumeRole err:" + err.Error())
        return nil, err
    }
    if resp == nil {
        fmt.Println("GetStsCredentials response is nil")
        return nil, err
    }
    var expire int64
    if resp.Credentials != nil && resp.Credentials.Expiration != nil {
        expire = resp.Credentials.Expiration.Unix()
    }
    return &StsCredentials{
        AccessKeyId:   *resp.Credentials.AccessKeyId,
        AccessSecret:  *resp.Credentials.SecretAccessKey,
        SecurityToken: *resp.Credentials.SessionToken,
        ExpireTime: expire,
    }, nil
}