[efi] Provide read-only access to EFI variables via settings mechanism

EFI variables do not map neatly to the iPXE settings mechanism, since
the EFI variable identifier includes a namespace GUID that cannot
cleanly be supplied as part of a setting name.  Creating a new EFI
variable requires the variable's attributes to be specified, which
does not fit within iPXE's settings concept.

However, EFI variable names are generally unique even without the
namespace GUID, and EFI does provide a mechanism to iterate over all
existent variables.  We can therefore provide read-only access to EFI
variables by comparing only the names and ignoring the namespace
GUIDs.

Provide an "efi" settings block that implements this mechanism using a
syntax such as:

  echo Platform language is ${efi/PlatformLang:string}

  show efi/SecureBoot:int8

Settings are returned as raw binary values by default since an EFI
variable may contain boolean flags, integer values, ASCII strings,
UCS-2 strings, EFI device paths, X.509 certificates, or any other
arbitrary blob of data.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/config/config.c

@@ -355,6 +355,9 @@ REQUIRE_OBJECT ( vram_settings );
 #ifdef ACPI_SETTINGS
 REQUIRE_OBJECT ( acpi_settings );
 #endif
+#ifdef EFI_SETTINGS
+REQUIRE_OBJECT ( efi_settings );
+#endif
 
 /*
  * Drag in selected keyboard map

src/config/defaults/efi.h

@@ -48,6 +48,8 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
 #define	REBOOT_CMD		/* Reboot command */
 
+#define EFI_SETTINGS		/* EFI variable settings */
+
 #if defined ( __i386__ ) || defined ( __x86_64__ )
 #define IOAPI_X86
 #define NAP_EFIX86

src/config/settings.h

@@ -9,6 +9,8 @@
 
 FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 
+#include <config/defaults.h>
+
 #define	PCI_SETTINGS	/* PCI device settings */
 //#define	CPUID_SETTINGS	/* CPUID settings */
 //#define	MEMMAP_SETTINGS	/* Memory map settings */

src/include/ipxe/errfile.h

@@ -406,6 +406,7 @@ FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
 #define ERRFILE_efi_cmdline	      ( ERRFILE_OTHER | 0x005b0000 )
 #define ERRFILE_efi_rng		      ( ERRFILE_OTHER | 0x005c0000 )
 #define ERRFILE_efi_shim	      ( ERRFILE_OTHER | 0x005d0000 )
+#define ERRFILE_efi_settings	      ( ERRFILE_OTHER | 0x005e0000 )
 
 /** @} */
 

src/interface/efi/efi_settings.c

@@ -0,0 +1,236 @@
+/*
+ * Copyright (C) 2023 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+/**
+ * @file
+ *
+ * EFI variable settings
+ *
+ */
+
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <ipxe/settings.h>
+#include <ipxe/init.h>
+#include <ipxe/efi/efi.h>
+#include <ipxe/efi/efi_strings.h>
+
+/** EFI variable settings scope */
+static const struct settings_scope efivars_scope;
+
+/** EFI variable settings */
+static struct settings efivars;
+
+/**
+ * Check applicability of EFI variable setting
+ *
+ * @v settings		Settings block
+ * @v setting		Setting
+ * @ret applies		Setting applies within this settings block
+ */
+static int efivars_applies ( struct settings *settings __unused,
+			     const struct setting *setting ) {
+
+	return ( setting->scope == &efivars_scope );
+}
+
+/**
+ * Find first matching EFI variable name
+ *
+ * @v wname		Name
+ * @v guid		GUID to fill in
+ * @ret rc		Return status code
+ */
+static int efivars_find ( const CHAR16 *wname, EFI_GUID *guid ) {
+	EFI_RUNTIME_SERVICES *rs = efi_systab->RuntimeServices;
+	size_t wname_len = ( ( wcslen ( wname ) + 1 ) * sizeof ( wname[0] ) );
+	CHAR16 *buf;
+	CHAR16 *tmp;
+	UINTN size;
+	EFI_STATUS efirc;
+	int rc;
+
+	/* Allocate single wNUL for first call to GetNextVariableName() */
+	size = sizeof ( buf[0] );
+	buf = zalloc ( size );
+	if ( ! buf )
+		return -ENOMEM;
+
+	/* Iterate over all veriables */
+	while ( 1 ) {
+
+		/* Get next variable name */
+		efirc = rs->GetNextVariableName ( &size, buf, guid );
+		if ( efirc == EFI_BUFFER_TOO_SMALL ) {
+			tmp = realloc ( buf, size );
+			if ( ! tmp ) {
+				rc = -ENOMEM;
+				break;
+			}
+			buf = tmp;
+			efirc = rs->GetNextVariableName ( &size, buf, guid );
+		}
+		if ( efirc == EFI_NOT_FOUND ) {
+			rc = -ENOENT;
+			break;
+		}
+		if ( efirc != 0 ) {
+			rc = -EEFI ( efirc );
+			DBGC ( &efivars, "EFIVARS %s:%ls could not fetch next "
+			       "variable name: %s\n",
+			       efi_guid_ntoa ( guid ), buf, strerror ( rc ) );
+			break;
+		}
+		DBGC2 ( &efivars, "EFIVARS %s:%ls exists\n",
+			efi_guid_ntoa ( guid ), buf );
+
+		/* Check for matching variable name */
+		if ( memcmp ( wname, buf, wname_len ) == 0 ) {
+			rc = 0;
+			break;
+		}
+	}
+
+	/* Free temporary buffer */
+	free ( buf );
+
+	return rc;
+}
+
+/**
+ * Fetch value of EFI variable setting
+ *
+ * @v settings		Settings block
+ * @v setting		Setting to fetch
+ * @v data		Buffer to fill with setting data
+ * @v len		Length of buffer
+ * @ret len		Length of setting data, or negative error
+ */
+static int efivars_fetch ( struct settings *settings __unused,
+			   struct setting *setting, void *data, size_t len ) {
+	EFI_RUNTIME_SERVICES *rs = efi_systab->RuntimeServices;
+	size_t name_len = strlen ( setting->name );
+	CHAR16 wname[ name_len + 1 /* wNUL */ ];
+	EFI_GUID guid;
+	UINT32 attrs;
+	UINTN size;
+	void *buf;
+	EFI_STATUS efirc;
+	int rc;
+
+	/* Convert name to UCS-2 */
+	efi_snprintf ( wname, sizeof ( wname ), "%s", setting->name );
+
+	/* Find variable GUID */
+	if ( ( rc = efivars_find ( wname, &guid ) ) != 0 )
+		goto err_find;
+
+	/* Get variable length */
+	size = 0;
+	if ( ( efirc = rs->GetVariable ( wname, &guid, &attrs, &size,
+					 NULL ) != EFI_BUFFER_TOO_SMALL ) ) {
+		rc = -EEFI ( efirc );
+		DBGC ( &efivars, "EFIVARS %s:%ls could not get size: %s\n",
+		       efi_guid_ntoa ( &guid ), wname, strerror ( rc ) );
+		goto err_len;
+	}
+
+	/* Allocate temporary buffer, since GetVariable() is not
+	 * guaranteed to return partial data for an underlength
+	 * buffer.
+	 */
+	buf = malloc ( size );
+	if ( ! buf ) {
+		rc = -ENOMEM;
+		goto err_alloc;
+	}
+
+	/* Get variable value */
+	if ( ( efirc = rs->GetVariable ( wname, &guid, &attrs, &size,
+					 buf ) ) != 0 ) {
+		rc = -EEFI ( efirc );
+		DBGC ( &efivars, "EFIVARS %s:%ls could not get %zd bytes: "
+		       "%s\n", efi_guid_ntoa ( &guid ), wname,
+		       ( ( size_t ) size ), strerror ( rc ) );
+		goto err_get;
+	}
+	DBGC ( &efivars, "EFIVARS %s:%ls:\n", efi_guid_ntoa ( &guid ), wname );
+	DBGC_HDA ( &efivars, 0, buf, size );
+
+	/* Return setting value */
+	if ( len > size )
+		len = size;
+	memcpy ( data, buf, len );
+	if ( ! setting->type )
+		setting->type = &setting_type_hex;
+
+	/* Free temporary buffer */
+	free ( buf );
+
+	return size;
+
+ err_get:
+	free ( buf );
+ err_alloc:
+ err_len:
+ err_find:
+	return rc;
+}
+
+/** EFI variable settings operations */
+static struct settings_operations efivars_operations = {
+	.applies = efivars_applies,
+	.fetch = efivars_fetch,
+};
+
+/** EFI variable settings */
+static struct settings efivars = {
+	.refcnt = NULL,
+	.siblings = LIST_HEAD_INIT ( efivars.siblings ),
+	.children = LIST_HEAD_INIT ( efivars.children ),
+	.op = &efivars_operations,
+	.default_scope = &efivars_scope,
+};
+
+/**
+ * Initialise EFI variable settings
+ *
+ */
+static void efivars_init ( void ) {
+	int rc;
+
+	/* Register settings block */
+	if ( ( rc = register_settings ( &efivars, NULL, "efi" ) ) != 0 ) {
+		DBGC ( &efivars, "EFIVARS could not register: %s\n",
+		       strerror ( rc ) );
+		return;
+	}
+}
+
+/** EFI variable settings initialiser */
+struct init_fn efivars_init_fn __init_fn ( INIT_NORMAL ) = {
+	.initialise = efivars_init,
+};