fix: Fixes the bucket policy json document validation errors for invalid jsons.

Fixes #965

Changes the returned error description to `Policies must be valid JSON and the first byte must be '{'` for invalid bucket policy json documents, which doesn't start with `{`.
The gateway returns `This policy contains invalid Json` error description, if the document starts with `{`, but still isn't valid json.

Implements the `policyErr` string type which implements the `error` interface, to handle the policy json document validation errors, by avoiding staticchecker warnings.

auth/bucket_policy.go

@@ -22,14 +22,20 @@ import (
 	"github.com/versity/versitygw/s3err"
 )
 
-var (
-	errResourceMismatch = errors.New("Action does not apply to any resource(s) in statement")
-	//lint:ignore ST1005 Reason: This error message is intended for end-user clarity and follows their expectations
-	errInvalidResource = errors.New("Policy has invalid resource")
-	//lint:ignore ST1005 Reason: This error message is intended for end-user clarity and follows their expectations
-	errInvalidPrincipal = errors.New("Invalid principal in policy")
-	//lint:ignore ST1005 Reason: This error message is intended for end-user clarity and follows their expectations
-	errInvalidAction = errors.New("Policy has invalid action")
+type policyErr string
+
+func (p policyErr) Error() string {
+	return string(p)
+}
+
+const (
+	policyErrResourceMismatch = policyErr("Action does not apply to any resource(s) in statement")
+	policyErrInvalidResource  = policyErr("Policy has invalid resource")
+	policyErrInvalidPrincipal = policyErr("Invalid principal in policy")
+	policyErrInvalidAction    = policyErr("Policy has invalid action")
+	policyErrInvalidPolicy    = policyErr("This policy contains invalid Json")
+	policyErrInvalidFirstChar = policyErr("Policies must be valid JSON and the first byte must be '{'")
+	policyErrEmptyStatement   = policyErr("Could not parse the policy: Statement is empty!")
 )
 
 type BucketPolicy struct {
@@ -90,10 +96,10 @@ func (bpi *BucketPolicyItem) Validate(bucket string, iam IAMService) error {
 			break
 		}
 		if *isObjectAction && !containsObjectAction {
-			return errResourceMismatch
+			return policyErrResourceMismatch
 		}
 		if !*isObjectAction && !containsBucketAction {
-			return errResourceMismatch
+			return policyErrResourceMismatch
 		}
 	}
 
@@ -117,14 +123,20 @@ func getMalformedPolicyError(err error) error {
 }
 
 func ValidatePolicyDocument(policyBin []byte, bucket string, iam IAMService) error {
+	if len(policyBin) == 0 || policyBin[0] != '{' {
+		return getMalformedPolicyError(policyErrInvalidFirstChar)
+	}
 	var policy BucketPolicy
 	if err := json.Unmarshal(policyBin, &policy); err != nil {
-		return getMalformedPolicyError(err)
+		var pe policyErr
+		if errors.As(err, &pe) {
+			return getMalformedPolicyError(err)
+		}
+		return getMalformedPolicyError(policyErrInvalidPolicy)
 	}
 
 	if len(policy.Statement) == 0 {
-		//lint:ignore ST1005 Reason: This error message is intended for end-user clarity and follows their expectations
-		return getMalformedPolicyError(errors.New("Could not parse the policy: Statement is empty!"))
+		return getMalformedPolicyError(policyErrEmptyStatement)
 	}
 
 	if err := policy.Validate(bucket, iam); err != nil {

auth/bucket_policy_actions.go

@@ -125,7 +125,7 @@ var supportedObjectActionList = map[Action]struct{}{
 // Validates Action: it should either wildcard match with supported actions list or be in it
 func (a Action) IsValid() error {
 	if !strings.HasPrefix(string(a), "s3:") {
-		return errInvalidAction
+		return policyErrInvalidAction
 	}
 
 	if a == AllActions {
@@ -140,12 +140,12 @@ func (a Action) IsValid() error {
 			}
 		}
 
-		return errInvalidAction
+		return policyErrInvalidAction
 	}
 
 	_, found := supportedActionList[a]
 	if !found {
-		return errInvalidAction
+		return policyErrInvalidAction
 	}
 	return nil
 }
@@ -191,7 +191,7 @@ func (a *Actions) UnmarshalJSON(data []byte) error {
 	var err error
 	if err = json.Unmarshal(data, &ss); err == nil {
 		if len(ss) == 0 {
-			return errInvalidAction
+			return policyErrInvalidAction
 		}
 		*a = make(Actions)
 		for _, s := range ss {
@@ -204,7 +204,7 @@ func (a *Actions) UnmarshalJSON(data []byte) error {
 		var s string
 		if err = json.Unmarshal(data, &s); err == nil {
 			if s == "" {
-				return errInvalidAction
+				return policyErrInvalidAction
 			}
 			*a = make(Actions)
 			err = a.Add(s)

auth/bucket_policy_principals.go

@@ -36,7 +36,7 @@ func (p *Principals) UnmarshalJSON(data []byte) error {
 
 	if err = json.Unmarshal(data, &ss); err == nil {
 		if len(ss) == 0 {
-			return errInvalidPrincipal
+			return policyErrInvalidPrincipal
 		}
 		*p = make(Principals)
 		for _, s := range ss {
@@ -45,7 +45,7 @@ func (p *Principals) UnmarshalJSON(data []byte) error {
 		return nil
 	} else if err = json.Unmarshal(data, &s); err == nil {
 		if s == "" {
-			return errInvalidPrincipal
+			return policyErrInvalidPrincipal
 		}
 		*p = make(Principals)
 		p.Add(s)
@@ -53,7 +53,7 @@ func (p *Principals) UnmarshalJSON(data []byte) error {
 		return nil
 	} else if err = json.Unmarshal(data, &k); err == nil {
 		if k.AWS == "" {
-			return errInvalidPrincipal
+			return policyErrInvalidPrincipal
 		}
 		*p = make(Principals)
 		p.Add(k.AWS)
@@ -65,7 +65,7 @@ func (p *Principals) UnmarshalJSON(data []byte) error {
 		}
 		if err = json.Unmarshal(data, &sk); err == nil {
 			if len(sk.AWS) == 0 {
-				return errInvalidPrincipal
+				return policyErrInvalidPrincipal
 			}
 			*p = make(Principals)
 			for _, s := range sk.AWS {
@@ -97,7 +97,7 @@ func (p Principals) Validate(iam IAMService) error {
 		if len(p) == 1 {
 			return nil
 		}
-		return errInvalidPrincipal
+		return policyErrInvalidPrincipal
 	}
 
 	accs, err := CheckIfAccountsExist(p.ToSlice(), iam)
@@ -105,7 +105,7 @@ func (p Principals) Validate(iam IAMService) error {
 		return err
 	}
 	if len(accs) > 0 {
-		return errInvalidPrincipal
+		return policyErrInvalidPrincipal
 	}
 
 	return nil

auth/bucket_policy_resources.go

@@ -29,7 +29,7 @@ func (r *Resources) UnmarshalJSON(data []byte) error {
 	var err error
 	if err = json.Unmarshal(data, &ss); err == nil {
 		if len(ss) == 0 {
-			return errInvalidResource
+			return policyErrInvalidResource
 		}
 		*r = make(Resources)
 		for _, s := range ss {
@@ -42,7 +42,7 @@ func (r *Resources) UnmarshalJSON(data []byte) error {
 		var s string
 		if err = json.Unmarshal(data, &s); err == nil {
 			if s == "" {
-				return errInvalidResource
+				return policyErrInvalidResource
 			}
 			*r = make(Resources)
 			err = r.Add(s)
@@ -59,7 +59,7 @@ func (r *Resources) UnmarshalJSON(data []byte) error {
 func (r Resources) Add(rc string) error {
 	ok, pattern := isValidResource(rc)
 	if !ok {
-		return errInvalidResource
+		return policyErrInvalidResource
 	}
 
 	r[pattern] = struct{}{}
@@ -93,7 +93,7 @@ func (r Resources) ContainsBucketPattern() bool {
 func (r Resources) Validate(bucket string) error {
 	for resource := range r {
 		if !strings.HasPrefix(resource, bucket) {
-			return errInvalidResource
+			return policyErrInvalidResource
 		}
 	}