osbase/efivarfs/boot.go

// Copyright The Monogon Project Authors.
// SPDX-License-Identifier: Apache-2.0

package efivarfs

import (
	"bytes"
	"encoding/binary"
	"errors"
	"fmt"
	"math"
	"strings"
)

type LoadOptionCategory uint8

const (
	// Boot entries belonging to the Boot category are normal boot entries.
	LoadOptionCategoryBoot LoadOptionCategory = 0x0
	// Boot entries belonging to the App category are not booted as part of
	// the normal boot order, but are only launched via menu or hotkey.
	// This category is optional for bootloaders to support, before creating
	// new boot entries of this category firmware support needs to be
	// confirmed.
	LoadOptionCategoryApp LoadOptionCategory = 0x1
)

// LoadOption contains information on a payload to be loaded by EFI.
type LoadOption struct {
	// Human-readable description of what this load option loads.
	// This is what's being shown by the firmware when selecting a boot option.
	Description string
	// If set, firmware will skip this load option when it is in BootOrder.
	// It is unspecificed whether this prevents the user from booting the entry
	// manually.
	Inactive bool
	// If set, this load option will not be shown in any menu for load option
	// selection. This does not affect other functionality.
	Hidden bool
	// Category contains the category of the load entry. The selected category
	// affects various firmware behaviors, see the individual value
	// descriptions for more information.
	Category LoadOptionCategory
	// Path to the UEFI PE executable to execute when this load option is being
	// loaded.
	FilePath DevicePath
	// ExtraPaths contains additional device paths with vendor-specific
	// behavior. Can generally be left empty.
	ExtraPaths []DevicePath
	// OptionalData gets passed as an argument to the executed PE executable.
	// If zero-length a NULL value is passed to the executable.
	OptionalData []byte
}

// Marshal encodes a LoadOption into a binary EFI_LOAD_OPTION.
func (e *LoadOption) Marshal() ([]byte, error) {
	var data []byte
	var attrs uint32
	attrs |= (uint32(e.Category) & 0x1f) << 8
	if e.Hidden {
		attrs |= 0x08
	}
	if !e.Inactive {
		attrs |= 0x01
	}
	data = append32(data, attrs)
	filePathRaw, err := e.FilePath.Marshal()
	if err != nil {
		return nil, fmt.Errorf("failed marshalling FilePath: %w", err)
	}
	for _, ep := range e.ExtraPaths {
		epRaw, err := ep.Marshal()
		if err != nil {
			return nil, fmt.Errorf("failed marshalling ExtraPath: %w", err)
		}
		filePathRaw = append(filePathRaw, epRaw...)
	}
	if len(filePathRaw) > math.MaxUint16 {
		return nil, fmt.Errorf("failed marshalling FilePath/ExtraPath: value too big (%d)", len(filePathRaw))
	}
	data = append16(data, uint16(len(filePathRaw)))
	if strings.IndexByte(e.Description, 0x00) != -1 {
		return nil, fmt.Errorf("failed to encode Description: contains invalid null bytes")
	}
	encodedDesc, err := Encoding.NewEncoder().Bytes([]byte(e.Description))
	if err != nil {
		return nil, fmt.Errorf("failed to encode Description: %w", err)
	}
	data = append(data, encodedDesc...)
	data = append(data, 0x00, 0x00) // Final UTF-16/UCS-2 null code
	data = append(data, filePathRaw...)
	data = append(data, e.OptionalData...)
	return data, nil
}

// UnmarshalLoadOption decodes a binary EFI_LOAD_OPTION into a LoadOption.
func UnmarshalLoadOption(data []byte) (*LoadOption, error) {
	if len(data) < 6 {
		return nil, fmt.Errorf("invalid load option: minimum 6 bytes are required, got %d", len(data))
	}
	var opt LoadOption
	attrs := binary.LittleEndian.Uint32(data[:4])
	opt.Category = LoadOptionCategory((attrs >> 8) & 0x1f)
	opt.Hidden = attrs&0x08 != 0
	opt.Inactive = attrs&0x01 == 0
	lenPath := binary.LittleEndian.Uint16(data[4:6])
	// Search for UTF-16 null code
	nullIdx := bytes.Index(data[6:], []byte{0x00, 0x00})
	if nullIdx == -1 {
		return nil, errors.New("no null code point marking end of Description found")
	}
	descriptionEnd := 6 + nullIdx + 1
	descriptionRaw := data[6:descriptionEnd]
	description, err := Encoding.NewDecoder().Bytes(descriptionRaw)
	if err != nil {
		return nil, fmt.Errorf("error decoding UTF-16 in Description: %w", err)
	}
	descriptionEnd += 2 // 2 null bytes terminating UTF-16 string
	opt.Description = string(description)
	if descriptionEnd+int(lenPath) > len(data) {
		return nil, fmt.Errorf("declared length of FilePath (%d) overruns available data (%d)", lenPath, len(data)-descriptionEnd)
	}
	filePathData := data[descriptionEnd : descriptionEnd+int(lenPath)]
	opt.FilePath, filePathData, err = UnmarshalDevicePath(filePathData)
	if err != nil {
		return nil, fmt.Errorf("failed unmarshaling FilePath: %w", err)
	}
	for len(filePathData) > 0 {
		var extraPath DevicePath
		extraPath, filePathData, err = UnmarshalDevicePath(filePathData)
		if err != nil {
			return nil, fmt.Errorf("failed unmarshaling ExtraPath: %w", err)
		}
		opt.ExtraPaths = append(opt.ExtraPaths, extraPath)
	}

	if descriptionEnd+int(lenPath) < len(data) {
		opt.OptionalData = data[descriptionEnd+int(lenPath):]
	}
	return &opt, nil
}

// BootOrder represents the contents of the BootOrder EFI variable.
type BootOrder []uint16

// Marshal generates the binary representation of a BootOrder.
func (t *BootOrder) Marshal() []byte {
	var out []byte
	for _, v := range *t {
		out = append16(out, v)
	}
	return out
}

// UnmarshalBootOrder loads a BootOrder from its binary representation.
func UnmarshalBootOrder(d []byte) (BootOrder, error) {
	if len(d)%2 != 0 {
		return nil, fmt.Errorf("invalid length: %v bytes", len(d))
	}
	l := len(d) / 2
	out := make(BootOrder, l)
	for i := 0; i < l; i++ {
		out[i] = uint16(d[2*i]) | uint16(d[2*i+1])<<8
	}
	return out, nil
}

func append16(d []byte, v uint16) []byte {
	return append(d,
		byte(v&0xFF),
		byte(v>>8&0xFF),
	)
}

func append32(d []byte, v uint32) []byte {
	return append(d,
		byte(v&0xFF),
		byte(v>>8&0xFF),
		byte(v>>16&0xFF),
		byte(v>>24&0xFF),
	)
}