shim: verify Extended Key Usage flags

For starters; don't allow the "module signing" OID; which ought to
only ever be used for signing kernel modules, not signing EFI binaries.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>

diff --git a/shim.c b/shim.c
index 533f5aa5c492629406d0b3d50dd9e35cbb720af6..c69961b99c4b141ab5aa5b1ce402e0bbf8119236 100644 (file)
--- a/shim.c
+++ b/shim.c
@@ -52,9 +52,15 @@
 #include "console.h"
 #include "version.h"
 
+#include <stdarg.h>
+#include <openssl/x509.h>
+#include <openssl/x509v3.h>
+
 #define FALLBACK L"\\fb" EFI_ARCH L".efi"
 #define MOK_MANAGER L"\\mm" EFI_ARCH L".efi"
 
+#define OID_EKU_MODSIGN "1.3.6.1.4.1.2312.16.1.2"
+
 static EFI_SYSTEM_TABLE *systab;
 static EFI_HANDLE image_handle;
 static EFI_STATUS (EFIAPI *entry_point) (EFI_HANDLE image_handle, EFI_SYSTEM_TABLE *system_table);
@@ -389,6 +395,38 @@ static BOOLEAN verify_x509(UINT8 *Cert, UINTN CertSize)
        return TRUE;
 }
 
+static BOOLEAN verify_eku(UINT8 *Cert, UINTN CertSize)
+{
+       X509 *x509;
+       CONST UINT8 *Temp = Cert;
+       EXTENDED_KEY_USAGE *eku;
+       ASN1_OBJECT *module_signing;
+
+       module_signing = OBJ_nid2obj(OBJ_create(OID_EKU_MODSIGN, NULL, NULL));
+
+       x509 = d2i_X509 (NULL, &Temp, (long) CertSize);
+       if (x509 != NULL) {
+               eku = X509_get_ext_d2i(x509, NID_ext_key_usage, NULL, NULL);
+
+               if (eku) {
+                       int i = 0;
+                       for (i = 0; i < sk_ASN1_OBJECT_num(eku); i++) {
+                               ASN1_OBJECT *key_usage = sk_ASN1_OBJECT_value(eku, i);
+
+                               if (OBJ_cmp(module_signing, key_usage) == 0)
+                                       return FALSE;
+                       }
+                       EXTENDED_KEY_USAGE_free(eku);
+               }
+
+               X509_free(x509);
+       }
+
+       OBJ_cleanup();
+
+       return TRUE;
+}
+
 static CHECK_STATUS check_db_cert_in_ram(EFI_SIGNATURE_LIST *CertList,
                                         UINTN dbsize,
                                         WIN_CERTIFICATE_EFI_PKCS *data,
@@ -404,13 +442,15 @@ static CHECK_STATUS check_db_cert_in_ram(EFI_SIGNATURE_LIST *CertList,
                        Cert = (EFI_SIGNATURE_DATA *) ((UINT8 *) CertList + sizeof (EFI_SIGNATURE_LIST) + CertList->SignatureHeaderSize);
                        CertSize = CertList->SignatureSize - sizeof(EFI_GUID);
                        if (verify_x509(Cert->SignatureData, CertSize)) {
-                               IsFound = AuthenticodeVerify (data->CertData,
-                                                             data->Hdr.dwLength - sizeof(data->Hdr),
-                                                             Cert->SignatureData,
-                                                             CertSize,
-                                                             hash, SHA256_DIGEST_SIZE);
-                               if (IsFound)
-                                       return DATA_FOUND;
+                               if (verify_eku(Cert->SignatureData, CertSize)) {
+                                       IsFound = AuthenticodeVerify (data->CertData,
+                                                                     data->Hdr.dwLength - sizeof(data->Hdr),
+                                                                     Cert->SignatureData,
+                                                                     CertSize,
+                                                                     hash, SHA256_DIGEST_SIZE);
+                                       if (IsFound)
+                                               return DATA_FOUND;
+                               }
                        } else if (verbose) {
                                console_notify(L"Not a DER encoding x.509 Certificate");
                        }