efi: Add a function to check if Secure Boot mode is enabled

Also cache it to avoid needing to repeatedly ask the firmware.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Gerald Elder-Vass <gerald.elder-vass@cloud.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/common/efi/boot.c

@@ -901,6 +901,28 @@ static void __init pre_parse(const struct file *file)
                    " last line will be ignored.\r\n");
 }
 
+static void __init init_secure_boot_mode(void)
+{
+    static EFI_GUID __initdata gv_uuid = EFI_GLOBAL_VARIABLE;
+    static CHAR16 __initdata str_SecureBoot[] = L"SecureBoot";
+    EFI_STATUS status;
+    uint8_t data = 0;
+    UINTN size = sizeof(data);
+    UINT32 attr = 0;
+
+    status = efi_rs->GetVariable(str_SecureBoot, &gv_uuid, &attr, &size, &data);
+
+    if ( status == EFI_NOT_FOUND ||
+         (status == EFI_SUCCESS &&
+          attr == (EFI_VARIABLE_BOOTSERVICE_ACCESS | EFI_VARIABLE_RUNTIME_ACCESS) &&
+          size == 1 && data == 0) )
+        /* Platform does not support Secure Boot or it's disabled. */
+        efi_secure_boot = false;
+    else
+        /* Everything else play it safe and assume enabled. */
+        efi_secure_boot = true;
+}
+
 static void __init efi_init(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
 {
     efi_ih = ImageHandle;
@@ -915,6 +937,8 @@ static void __init efi_init(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTabl
 
     StdOut = SystemTable->ConOut;
     StdErr = SystemTable->StdErr ?: StdOut;
+
+    init_secure_boot_mode();
 }
 
 static void __init efi_console_set_mode(void)

xen/common/efi/runtime.c

@@ -41,6 +41,7 @@ void efi_rs_leave(struct efi_rs_state *state);
 unsigned int __read_mostly efi_num_ct;
 const EFI_CONFIGURATION_TABLE *__read_mostly efi_ct;
 
+bool __ro_after_init efi_secure_boot;
 unsigned int __read_mostly efi_version;
 unsigned int __read_mostly efi_fw_revision;
 const CHAR16 *__read_mostly efi_fw_vendor;

xen/include/xen/efi.h

@@ -36,6 +36,8 @@ static inline bool efi_enabled(unsigned int feature)
 }
 #endif
 
+extern bool efi_secure_boot;
+
 void efi_init_memory(void);
 bool efi_boot_mem_unused(unsigned long *start, unsigned long *end);
 bool efi_rs_using_pgtables(void);