fix: hardcode azure instance identity intermediate certificates (#6402)

This was broken for an airgapped customer!

coderd/azureidentity/azureidentity.go

@@ -1,12 +1,10 @@
 package azureidentity
 
 import (
-	"context"
 	"crypto/x509"
 	"encoding/base64"
 	"encoding/json"
-	"io"
-	"net/http"
+	"encoding/pem"
 	"regexp"
 
 	"go.mozilla.org/pkcs7"
@@ -23,7 +21,7 @@ type metadata struct {
 
 // Validate ensures the signature was signed by an Azure certificate.
 // It returns the associated VM ID if successful.
-func Validate(ctx context.Context, signature string, options x509.VerifyOptions) (string, error) {
+func Validate(signature string, options x509.VerifyOptions) (string, error) {
 	data, err := base64.StdEncoding.DecodeString(signature)
 	if err != nil {
 		return "", xerrors.Errorf("decode base64: %w", err)
@@ -41,24 +39,14 @@ func Validate(ctx context.Context, signature string, options x509.VerifyOptions)
 	}
 	if options.Intermediates == nil {
 		options.Intermediates = x509.NewCertPool()
-		for _, certURL := range signer.IssuingCertificateURL {
-			req, err := http.NewRequestWithContext(ctx, "GET", certURL, nil)
-			if err != nil {
-				return "", xerrors.Errorf("new request %q: %w", certURL, err)
+		for _, cert := range certificates {
+			block, rest := pem.Decode([]byte(cert))
+			if len(rest) != 0 {
+				return "", xerrors.Errorf("invalid certificate. %d bytes remain", len(rest))
 			}
-			res, err := http.DefaultClient.Do(req)
+			cert, err := x509.ParseCertificate(block.Bytes)
 			if err != nil {
-				return "", xerrors.Errorf("perform request %q: %w", certURL, err)
-			}
-			data, err := io.ReadAll(res.Body)
-			if err != nil {
-				_ = res.Body.Close()
-				return "", xerrors.Errorf("read body %q: %w", certURL, err)
-			}
-			_ = res.Body.Close()
-			cert, err := x509.ParseCertificate(data)
-			if err != nil {
-				return "", xerrors.Errorf("parse certificate %q: %w", certURL, err)
+				return "", xerrors.Errorf("parse certificate: %w", err)
 			}
 			options.Intermediates.AddCert(cert)
 		}
@@ -76,3 +64,68 @@ func Validate(ctx context.Context, signature string, options x509.VerifyOptions)
 	}
 	return metadata.VMID, nil
 }
+
+var certificates = []string{
+	`-----BEGIN CERTIFICATE-----
+MIIFWjCCBEKgAwIBAgIQDxSWXyAgaZlP1ceseIlB4jANBgkqhkiG9w0BAQsFADBa
+MQswCQYDVQQGEwJJRTESMBAGA1UEChMJQmFsdGltb3JlMRMwEQYDVQQLEwpDeWJl
+clRydXN0MSIwIAYDVQQDExlCYWx0aW1vcmUgQ3liZXJUcnVzdCBSb290MB4XDTIw
+MDcyMTIzMDAwMFoXDTI0MTAwODA3MDAwMFowTzELMAkGA1UEBhMCVVMxHjAcBgNV
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+IMW/snBjXXgMQ8ApzWRJCZbar879BV8rKpHoAW4uGJssnNABf2n17j9TiFy6BWy+
+IhVnFILyLNK+W2M3zK9gheiWa2uACKhuvgCca5Vw/OQYErEdG7LBEzFnMzTmJcli
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+cGHTBPK4R28F44qIMCtHRV55VMX53ev6P3hRddJb
+-----END CERTIFICATE-----`,
+	`-----BEGIN CERTIFICATE-----
+MIIFWjCCBEKgAwIBAgIQD6dHIsU9iMgPWJ77H51KOjANBgkqhkiG9w0BAQsFADBa
+MQswCQYDVQQGEwJJRTESMBAGA1UEChMJQmFsdGltb3JlMRMwEQYDVQQLEwpDeWJl
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+lk+521W+Ncih3HRbvRBE0LWYT8vWKnfjgZKxwHwJ
+-----END CERTIFICATE-----`,
+}