Fix time formatting of Expiration field in STS (#8043)

Without explicit conversion to UTC() from Unix
time the zone information is lost, this leads
to XML marshallers marshaling the time into
a wrong format.

This PR fixes the compatibility issue with AWS STS
API by keeping Expiration format close to ISO8601
or RFC3339

Fixes #8041

docs/sts/assume-role.md

@@ -59,7 +59,7 @@ http://minio:9000/?Action=AssumeRole&DurationSeconds=3600&Version=2011-06-15&Pol
     <Credentials>
       <AccessKeyId>Y4RJU1RNFGK48LGO9I2S</AccessKeyId>
       <SecretAccessKey>sYLRKS1Z7hSjluf6gEbb9066hnx315wHTiACPAjg</SecretAccessKey>
-      <Expiration>2018-11-09T16:51:11-08:00</Expiration>
+      <Expiration>2019-08-08T20:26:12Z</Expiration>
       <SessionToken>eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJhY2Nlc3NLZXkiOiJZNFJKVTFSTkZHSzQ4TEdPOUkyUyIsImF1ZCI6IlBvRWdYUDZ1Vk80NUlzRU5SbmdEWGo1QXU1WWEiLCJhenAiOiJQb0VnWFA2dVZPNDVJc0VOUm5nRFhqNUF1NVlhIiwiZXhwIjoxNTQxODExMDcxLCJpYXQiOjE1NDE4MDc0NzEsImlzcyI6Imh0dHBzOi8vbG9jYWxob3N0Ojk0NDMvb2F1dGgyL3Rva2VuIiwianRpIjoiYTBiMjc2MjktZWUxYS00M2JmLTg3MzktZjMzNzRhNGNkYmMwIn0.ewHqKVFTaP-j_kgZrcOEKroNUjk10GEp8bqQjxBbYVovV0nHO985VnRESFbcT6XMDDKHZiWqN2vi_ETX_u3Q-w</SessionToken>
     </Credentials>
   </AssumeRoleResult>

docs/sts/client-grants.md

@@ -64,7 +64,7 @@ http://minio.cluster:9000?Action=AssumeRoleWithClientGrants&DurationSeconds=3600
     <Credentials>
       <AccessKeyId>Y4RJU1RNFGK48LGO9I2S</AccessKeyId>
       <SecretAccessKey>sYLRKS1Z7hSjluf6gEbb9066hnx315wHTiACPAjg</SecretAccessKey>
-      <Expiration>2018-11-09T16:51:11-08:00</Expiration>
+      <Expiration>2019-08-08T20:26:12Z</Expiration>
       <SessionToken>eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJhY2Nlc3NLZXkiOiJZNFJKVTFSTkZHSzQ4TEdPOUkyUyIsImF1ZCI6IlBvRWdYUDZ1Vk80NUlzRU5SbmdEWGo1QXU1WWEiLCJhenAiOiJQb0VnWFA2dVZPNDVJc0VOUm5nRFhqNUF1NVlhIiwiZXhwIjoxNTQxODExMDcxLCJpYXQiOjE1NDE4MDc0NzEsImlzcyI6Imh0dHBzOi8vbG9jYWxob3N0Ojk0NDMvb2F1dGgyL3Rva2VuIiwianRpIjoiYTBiMjc2MjktZWUxYS00M2JmLTg3MzktZjMzNzRhNGNkYmMwIn0.ewHqKVFTaP-j_kgZrcOEKroNUjk10GEp8bqQjxBbYVovV0nHO985VnRESFbcT6XMDDKHZiWqN2vi_ETX_u3Q-w</SessionToken>
     </Credentials>
   </AssumeRoleWithClientGrantsResult>

docs/sts/web-identity.md

@@ -62,7 +62,7 @@ http://minio.cluster:9000?Action=AssumeRoleWithWebIdentity&DurationSeconds=3600&
     <Credentials>
       <AccessKeyId>Y4RJU1RNFGK48LGO9I2S</AccessKeyId>
       <SecretAccessKey>sYLRKS1Z7hSjluf6gEbb9066hnx315wHTiACPAjg</SecretAccessKey>
-      <Expiration>2018-11-09T16:51:11-08:00</Expiration>
+      <Expiration>2019-08-08T20:26:12Z</Expiration>
       <SessionToken>eyJhbGciOiJIUzUxMiIsInR5cCI6IkpXVCJ9.eyJhY2Nlc3NLZXkiOiJZNFJKVTFSTkZHSzQ4TEdPOUkyUyIsImF1ZCI6IlBvRWdYUDZ1Vk80NUlzRU5SbmdEWGo1QXU1WWEiLCJhenAiOiJQb0VnWFA2dVZPNDVJc0VOUm5nRFhqNUF1NVlhIiwiZXhwIjoxNTQxODExMDcxLCJpYXQiOjE1NDE4MDc0NzEsImlzcyI6Imh0dHBzOi8vbG9jYWxob3N0Ojk0NDMvb2F1dGgyL3Rva2VuIiwianRpIjoiYTBiMjc2MjktZWUxYS00M2JmLTg3MzktZjMzNzRhNGNkYmMwIn0.ewHqKVFTaP-j_kgZrcOEKroNUjk10GEp8bqQjxBbYVovV0nHO985VnRESFbcT6XMDDKHZiWqN2vi_ETX_u3Q-w</SessionToken>
     </Credentials>
   </AssumeRoleWithWebIdentityResult>

pkg/auth/credentials.go

@@ -170,7 +170,7 @@ func GetNewCredentialsWithMetadata(m map[string]interface{}, tokenSecret string)
 	m["accessKey"] = cred.AccessKey
 	jwt := jwtgo.NewWithClaims(jwtgo.SigningMethodHS512, jwtgo.MapClaims(m))
 
-	cred.Expiration = time.Unix(expiry, 0)
+	cred.Expiration = time.Unix(expiry, 0).UTC()
 	cred.SessionToken, err = jwt.SignedString([]byte(tokenSecret))
 	if err != nil {
 		return cred, err