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// Copyright 2020-2021 Canonical Ltd.
// Licensed under the LGPLv3 with static-linking exception.
// See LICENCE file for details.
package efi
import (
"bytes"
"context"
"encoding/binary"
"errors"
"fmt"
"io"
"os"
"path/filepath"
"syscall"
"golang.org/x/sys/unix"
internal_unix "github.com/canonical/go-efilib/internal/unix"
)
func efivarfsPath() string {
return "/sys/firmware/efi/efivars"
}
type varFile interface {
io.ReadWriteCloser
Readdir(n int) ([]os.FileInfo, error)
Stat() (os.FileInfo, error)
GetInodeFlags() (uint, error)
SetInodeFlags(flags uint) error
}
func makeVarFileMutableAndTakeFile(f varFile) (restore func() error, err error) {
const immutableFlag = 0x00000010
flags, err := f.GetInodeFlags()
if err != nil {
return nil, err
}
if flags&immutableFlag == 0 {
// Nothing to do
f.Close()
return func() error { return nil }, nil
}
if err := f.SetInodeFlags(flags &^ immutableFlag); err != nil {
return nil, err
}
return func() error {
defer func() {
f.Close()
}()
return f.SetInodeFlags(flags)
}, nil
}
type realVarFile struct {
*os.File
}
func (f *realVarFile) GetInodeFlags() (uint, error) {
flags, err := internal_unix.IoctlGetUint(int(f.Fd()), unix.FS_IOC_GETFLAGS)
if err != nil {
return 0, &os.PathError{Op: "ioctl", Path: f.Name(), Err: err}
}
return flags, nil
}
func (f *realVarFile) SetInodeFlags(flags uint) error {
if err := internal_unix.IoctlSetPointerUint(int(f.Fd()), unix.FS_IOC_SETFLAGS, flags); err != nil {
return &os.PathError{Op: "ioctl", Path: f.Name(), Err: err}
}
return nil
}
var openVarFile = func(path string, flags int, perm os.FileMode) (varFile, error) {
f, err := os.OpenFile(path, flags, perm)
if err != nil {
return nil, err
}
return &realVarFile{f}, nil
}
var guidLength = len("xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx")
func probeEfivarfs() bool {
var st unix.Statfs_t
if err := unixStatfs(efivarfsPath(), &st); err != nil {
return false
}
if uint(st.Type) != uint(unix.EFIVARFS_MAGIC) {
return false
}
return true
}
func maybeRetry(n int, fn func() (bool, error)) error {
for i := 1; ; i++ {
retry, err := fn()
switch {
case i > n:
return err
case !retry:
return err
case err == nil:
return nil
}
}
}
// inodeMayBeImmutable returns whether the supplied error returned from open (for
// writing) or unlink indicates that the inode is immutable. This is indicated
// when the error is EPERM.
//
// We retry for EPERM errors that occur when opening an inode for writing,
// or when unlinking its directory entry. Although we temporarily mark inodes as
// mutable before opening them to write or when unlinking, we can get this error
// as a result of race with another process that might have been writing to the
// variable (and subsequently marked the inode as immutable again when it
// finished) or may have deleted and recreated it, making the new inode immutable.
func inodeMayBeImmutable(err error) bool {
var errno syscall.Errno
if !errors.As(err, &errno) {
return false
}
return errno == syscall.EPERM
}
func transformEfivarfsError(err error) error {
switch {
case errors.Is(err, os.ErrNotExist) || err == io.EOF:
// ENOENT can come from the VFS layer during opening and unlinking, and
// converted from EFI_NOT_FOUND errors returned from the variable
// service. When reading a variable, if the variable doesn't exist
// when trying to determine the size of it, the kernel converts ENOENT
// into success with 0 bytes read which means we need to handle io.EOF
// as well.
return ErrVarNotExist
case errors.Is(err, syscall.EINVAL):
// EINVAL can come from the VFS layer during opening or unlinking due
// to invalid or incompatible flag combinations, although we don't expect
// that. It's also converted from EFI_INVALID_PARAMETER errors returned
// from the variable service.
return ErrVarInvalidParam
case errors.Is(err, syscall.EIO):
// EIO can come from the VFS layer during unlinking, although we don't
// expect that. It's also converted from EFI_DEVICE_ERROR errors returned
// from the variable service
return ErrVarDeviceError
case errors.Is(err, os.ErrPermission):
// EACCESS can come from the VFS layer for the following reasons:
// - opening a file for writing when the caller does not have write
// access to it.
// - opening a file for writing when the caller does not have write
// access to the parent directory and a new file needs to be created.
// - unlinking a file when the caller does not have write access to
// the parent directory.
// EPERM can come from the VFS layer when opening a file for writing
// or unlinking if the inode is immutable (see inodeMayBeImmutable).
//
// EACCES is also converted from EFI_SECURITY_VIOLATION errors returned
// from the variable service.
return ErrVarPermission
case errors.Is(err, syscall.ENOSPC):
// ENOSPC is converted from EFI_OUT_OF_RESOURCES errors returned from
// the variable service.
return ErrVarInsufficientSpace
case errors.Is(err, syscall.EROFS):
// EROFS is converted from EFI_WRITE_PROTECTED errors returned from the
// variable service.
return ErrVarWriteProtected
default:
return err
}
}
func writeEfivarfsFile(path string, attrs VariableAttributes, data []byte) (retry bool, err error) {
// Open for reading to make the inode mutable
r, err := openVarFile(path, os.O_RDONLY, 0)
switch {
case errors.Is(err, os.ErrNotExist):
// It's not an error if the variable doesn't exist.
case err != nil:
return false, transformEfivarfsError(err)
default:
restoreImmutable, err := makeVarFileMutableAndTakeFile(r)
if err != nil {
r.Close()
return false, transformEfivarfsError(err)
}
defer restoreImmutable()
}
if len(data) == 0 {
// short-cut for unauthenticated variable delete - efivarfs will perform a
// zero-byte write to delete the variable if we unlink the entry here.
if attrs&(AttributeAuthenticatedWriteAccess|AttributeTimeBasedAuthenticatedWriteAccess|AttributeEnhancedAuthenticatedAccess) > 0 {
// If the supplied attributes are incompatible with the variable,
// the variable service will return EFI_INVALID_PARAMETER and
// we'll get EINVAL back. If the supplied attributes are correct
// but we perform a zero-byte write to an authenticated variable,
// the variable service will return EFI_SECURITY_VIOLATION, but
// the kernel also turns this into EINVAL. Instead, we generate
// an appropriate error if the supplied attributes indicate that
// the variable is authenticated.
return false, ErrVarPermission
}
if err := removeVarFile(path); err != nil {
switch {
case errors.Is(err, os.ErrNotExist):
// It's not an error if the variable doesn't exist.
return false, nil
case inodeMayBeImmutable(err):
// Try again
return true, transformEfivarfsError(err)
default:
// Don't try again
return false, transformEfivarfsError(err)
}
}
return false, nil
}
flags := os.O_WRONLY | os.O_CREATE
if attrs&AttributeAppendWrite != 0 {
flags |= os.O_APPEND
}
w, err := openVarFile(path, flags, 0644)
switch {
case inodeMayBeImmutable(err):
// Try again
return true, transformEfivarfsError(err)
case err != nil:
// Don't try again
return false, transformEfivarfsError(err)
}
defer w.Close()
var buf bytes.Buffer
binary.Write(&buf, binary.LittleEndian, attrs)
buf.Write(data)
_, err = buf.WriteTo(w)
return false, transformEfivarfsError(err)
}
type efivarfsVarsBackend struct{}
func (v efivarfsVarsBackend) Get(name string, guid GUID) (VariableAttributes, []byte, error) {
path := filepath.Join(efivarfsPath(), fmt.Sprintf("%s-%s", name, guid))
f, err := openVarFile(path, os.O_RDONLY, 0)
if err != nil {
return 0, nil, transformEfivarfsError(err)
}
defer f.Close()
// Read the entire payload in a single read, as that's how
// GetVariable works and is the only way the kernel can obtain
// the variable contents. If we perform multiple reads, the
// kernel still has to obtain the entire variable contents
// each time. To do this, we need to know the size of the variable
// contents, which we can obtain from the inode.
fi, err := f.Stat()
if err != nil {
return 0, nil, err
}
if fi.Size() < 4 {
return 0, nil, ErrVarNotExist
}
buf := make([]byte, fi.Size())
if _, err := f.Read(buf); err != nil {
return 0, nil, transformEfivarfsError(err)
}
return VariableAttributes(binary.LittleEndian.Uint32(buf)), buf[4:], nil
}
func (v efivarfsVarsBackend) Set(name string, guid GUID, attrs VariableAttributes, data []byte) error {
path := filepath.Join(efivarfsPath(), fmt.Sprintf("%s-%s", name, guid))
return maybeRetry(4, func() (bool, error) { return writeEfivarfsFile(path, attrs, data) })
}
func (v efivarfsVarsBackend) List() ([]VariableDescriptor, error) {
f, err := openVarFile(efivarfsPath(), os.O_RDONLY, 0)
switch {
case errors.Is(err, os.ErrNotExist):
return nil, ErrVarsUnavailable
case err != nil:
return nil, transformEfivarfsError(err)
}
defer f.Close()
dirents, err := f.Readdir(-1)
if err != nil {
return nil, err
}
var entries []VariableDescriptor
for _, dirent := range dirents {
if !dirent.Mode().IsRegular() {
// Skip non-regular files
continue
}
if len(dirent.Name()) < guidLength+1 {
// Skip files with a basename that isn't long enough
// to contain a GUID and a hyphen
continue
}
if dirent.Name()[len(dirent.Name())-guidLength-1] != '-' {
// Skip files where the basename doesn't contain a
// hyphen between the name and GUID
continue
}
if dirent.Size() == 0 {
// Skip files with zero size. These are variables that
// have been deleted by writing an empty payload
continue
}
name := dirent.Name()[:len(dirent.Name())-guidLength-1]
guid, err := DecodeGUIDString(dirent.Name()[len(name)+1:])
if err != nil {
continue
}
entries = append(entries, VariableDescriptor{Name: name, GUID: guid})
}
return entries, nil
}
func addDefaultVarsBackend(ctx context.Context) context.Context {
if !probeEfivarfs() {
return withVarsBackend(ctx, nullVarsBackend{})
}
return withVarsBackend(ctx, efivarfsVarsBackend{})
}
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