Fix: preserve request body for STS signature verification (#8324)

* Fix: preserve request body for STS signature verification

- Save and restore request body in UnifiedPostHandler after ParseForm()
- This allows STS handler to verify signatures correctly
- Fixes 'invalid AWS signature: 53' error (ErrContentSHA256Mismatch)
- ParseForm() consumes the body, so we need to restore it for downstream handlers

* Improve error handling in UnifiedPostHandler

- Add http.MaxBytesReader to limit body size to 10 MiB (iamRequestBodyLimit)
- Add proper error handling for io.ReadAll failures
- Log errors when body reading fails
- Prevents DoS attacks from oversized request bodies
- Addresses code review feedback

weed/s3api/s3api_server.go

@@ -1,9 +1,11 @@
 package s3api
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net"
 	"net/http"
 	"os"
@@ -435,11 +437,33 @@ func (s3a *S3ApiServer) UnifiedPostHandler(w http.ResponseWriter, r *http.Reques
 	}
 
 	// 2. Parse Form to get Action
+	// Save the body first so we can restore it for STS handler signature verification
+	var bodyBytes []byte
+	if r.Body != nil {
+		// Limit body size to prevent DoS attacks
+		r.Body = http.MaxBytesReader(w, r.Body, iamRequestBodyLimit)
+		var err error
+		bodyBytes, err = io.ReadAll(r.Body)
+		if err != nil {
+			glog.Errorf("failed to read request body: %v", err)
+			s3err.WriteErrorResponse(w, r, s3err.ErrInvalidRequest)
+			return
+		}
+		r.Body.Close()
+		// Restore body for ParseForm
+		r.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
+	}
+
 	if err := r.ParseForm(); err != nil {
 		s3err.WriteErrorResponse(w, r, s3err.ErrInvalidRequest)
 		return
 	}
 
+	// Restore body again for downstream handlers (STS needs it for signature verification)
+	if bodyBytes != nil {
+		r.Body = io.NopCloser(bytes.NewBuffer(bodyBytes))
+	}
+
 	// 3. Dispatch
 	action := r.Form.Get("Action")
 	if strings.HasPrefix(action, "AssumeRole") {