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efi: Make use of the grub's prefix for detection
Avoid relying on the signature in order to detect an Ubuntu production grub binary for binaries without a SBAT section, as this breaks test cases in snapd where grub is re-signed. Instead, obtain the prefix from the "mods" section (which is set to "/EFI/ubuntu" in Ubuntu).
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,158 @@ | ||
| // -*- Mode: Go; indent-tabs-mode: t -*- | ||
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| /* | ||
| * Copyright (C) 2023 Canonical Ltd | ||
| * | ||
| * This program is free software: you can redistribute it and/or modify | ||
| * it under the terms of the GNU General Public License version 3 as | ||
| * published by the Free Software Foundation. | ||
| * | ||
| * 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, see <http://www.gnu.org/licenses/>. | ||
| * | ||
| */ | ||
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| package efi | ||
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| import ( | ||
| "encoding/binary" | ||
| "errors" | ||
| "fmt" | ||
| "io" | ||
| "math" | ||
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| pe "github.com/snapcore/secboot/internal/pe1.14" | ||
| ) | ||
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| const grubObjTypePrefix uint32 = 3 | ||
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| type grubModuleHeader struct { | ||
| Type uint32 | ||
| Size uint32 | ||
| } | ||
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| const grubModuleMagic uint32 = 0x676d696d | ||
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| type grubModuleInfo32 struct { | ||
| Magic uint32 | ||
| Offset uint32 | ||
| Size uint32 | ||
| } | ||
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| type grubModuleInfo64 struct { | ||
| Magic uint32 | ||
| Padding uint32 | ||
| Offset uint64 | ||
| Size uint64 | ||
| } | ||
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| type grubModule struct { | ||
| Type uint32 | ||
| *io.SectionReader | ||
| } | ||
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| type grubImageHandle interface { | ||
| peImageHandle | ||
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| Prefix() (string, error) | ||
| } | ||
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| type grubImageHandleImpl struct { | ||
| peImageHandle | ||
| } | ||
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| // newGrubImageHandle returns a new grubImageHandle for the supplied peImageHandle. | ||
| var newGrubImageHandle = func(image peImageHandle) grubImageHandle { | ||
| return &grubImageHandleImpl{peImageHandle: image} | ||
| } | ||
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| func (h *grubImageHandleImpl) mods() ([]grubModule, error) { | ||
| section := h.OpenSection("mods") | ||
| if section == nil { | ||
| return nil, errors.New("no mods section") | ||
| } | ||
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| var r *io.SectionReader | ||
| switch h.Machine() { | ||
| case pe.IMAGE_FILE_MACHINE_AMD64, pe.IMAGE_FILE_MACHINE_ARM64, pe.IMAGE_FILE_MACHINE_RISCV64: | ||
| var info grubModuleInfo64 | ||
| if err := binary.Read(section, binary.LittleEndian, &info); err != nil { | ||
| return nil, fmt.Errorf("cannot obtain modules info: %w", err) | ||
| } | ||
| if info.Magic != grubModuleMagic { | ||
| return nil, errors.New("invalid modules magic") | ||
| } | ||
| if info.Size > math.MaxInt64 { | ||
| return nil, errors.New("invalid modules offset") | ||
| } | ||
| if info.Size > math.MaxInt64 || info.Size < uint64(binary.Size(info)) { | ||
| return nil, errors.New("invalid modules size") | ||
| } | ||
| r = io.NewSectionReader(section, int64(info.Offset), int64(info.Size)-int64(binary.Size(info))) | ||
| case pe.IMAGE_FILE_MACHINE_ARM, pe.IMAGE_FILE_MACHINE_I386, pe.IMAGE_FILE_MACHINE_RISCV32: | ||
| var info grubModuleInfo32 | ||
| if err := binary.Read(section, binary.LittleEndian, &info); err != nil { | ||
| return nil, fmt.Errorf("cannot obtain modules info: %w", err) | ||
| } | ||
| if info.Magic != grubModuleMagic { | ||
| return nil, errors.New("invalid module magic") | ||
| } | ||
| if info.Size < uint32(binary.Size(info.Size)) { | ||
| return nil, errors.New("invalid modules size") | ||
| } | ||
| r = io.NewSectionReader(section, int64(info.Offset), int64(info.Size)-int64(binary.Size(info))) | ||
| default: | ||
| return nil, fmt.Errorf("unrecognized machine: %d", h.Machine()) | ||
| } | ||
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| var mods []grubModule | ||
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| for { | ||
| var hdr grubModuleHeader | ||
| if err := binary.Read(r, binary.LittleEndian, &hdr); err != nil { | ||
| if err == io.EOF { | ||
| break | ||
| } | ||
| return nil, fmt.Errorf("cannot obtain module header: %w", err) | ||
| } | ||
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| offset, _ := r.Seek(0, io.SeekCurrent) | ||
| size := int64(hdr.Size) - int64(binary.Size(hdr)) | ||
| mods = append(mods, grubModule{ | ||
| Type: hdr.Type, | ||
| SectionReader: io.NewSectionReader(r, offset, size), | ||
| }) | ||
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| if _, err := r.Seek(size, io.SeekCurrent); err != nil { | ||
| return nil, fmt.Errorf("cannot seek to next module: %w", err) | ||
| } | ||
| } | ||
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| return mods, nil | ||
| } | ||
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| func (h *grubImageHandleImpl) Prefix() (string, error) { | ||
| mods, err := h.mods() | ||
| if err != nil { | ||
| return "", err | ||
| } | ||
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| for _, mod := range mods { | ||
| if mod.Type != grubObjTypePrefix { | ||
| continue | ||
| } | ||
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| prefix, err := io.ReadAll(newCstringReader(mod)) | ||
| if err != nil { | ||
| return "", fmt.Errorf("cannot obtain prefix: %w", err) | ||
| } | ||
| return string(prefix), nil | ||
| } | ||
|
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| return "", nil | ||
| } |
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