boot: check if boot assets are trusted by UEFI

boot/asset_trust_sb.go

@@ -0,0 +1,34 @@
+// -*- Mode: Go; indent-tabs-mode: t -*-
+//go:build !nosecboot
+
+/*
+ * Copyright (C) 2026 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/>.
+ *
+ */
+
+package boot
+
+import (
+	efi "github.com/canonical/go-efilib"
+	sb_efi "github.com/snapcore/secboot/efi"
+)
+
+func init() {
+	checkBootAssetAgainstSignatureDB = checkImageSignatureIsValidForHost
+}
+
+func checkImageSignatureIsValidForHost(assetPath string) error {
+	return sb_efi.CheckImageSignatureIsValidForHost(efi.DefaultVarContext, sb_efi.FileImage(assetPath))
+}

boot/assets.go

@@ -41,6 +41,15 @@ import (
 	"github.com/snapcore/snapd/strutil"
 )
 
+// checkBootAssetAgainstSignatureDB, when set, validates that the boot asset at
+// the given file path has at least one secure-boot signature that chains to a
+// certificate authority in the host's UEFI authorized signature database (db).
+// This prevents installing boot assets (e.g. shim) that the firmware will not
+// be able to verify, which would result in an unbootable system.
+//
+// This is wired to the secboot package when secboot support is available.
+var checkBootAssetAgainstSignatureDB func(assetPath string) error
+
 type trustedAssetsCache struct {
 	cacheDir string
 	hash     crypto.Hash
@@ -677,6 +686,12 @@ func (o *TrustedAssetsUpdateObserver) observeUpdate(bl bootloader.Bootloader, re
 		}
 	}
 
+	if checkBootAssetAgainstSignatureDB != nil {
+		if err := checkBootAssetAgainstSignatureDB(change.After); err != nil {
+			return gadget.ChangeAbort, fmt.Errorf("cannot use boot asset %q: %v", trustedAssetName, err)
+		}
+	}
+
 	ta, err := o.cache.Add(change.After, bl.Name(), trustedAssetName)
 	if err != nil {
 		return gadget.ChangeAbort, err

boot/assets_test.go

@@ -58,6 +58,14 @@ func (s *assetsSuite) SetUpTest(c *C) {
 	})
 	s.AddCleanup(restore)
 
+	// Most tests use synthetic boot assets (eg plain "C", "D", "shim") that
+	// are not valid signed EFI binaries. Keep signature validation permissive by
+	// default and let dedicated tests override this behavior explicitly.
+	restore = boot.MockCheckBootAssetAgainstSignatureDB(func(assetPath string) error {
+		return nil
+	})
+	s.AddCleanup(restore)
+
 	c.Assert(os.MkdirAll(boot.InitramfsUbuntuBootDir, 0755), IsNil)
 	c.Assert(os.MkdirAll(boot.InitramfsUbuntuSeedDir, 0755), IsNil)
 
@@ -3144,3 +3152,90 @@ func (s *assetsSuite) TestUpdateBootEntryOnInstall(c *C) {
 	c.Check(foundAsset2, Equals, 1)
 	c.Check(foundOther, Equals, 0)
 }
+
+func (s *assetsSuite) TestUpdateObserverUpdateRejectsUntrustedAsset(c *C) {
+	// Test that the observer rejects an update when the new boot asset's
+	// signature is not trusted by the host's UEFI signature database.
+
+	d := c.MkDir()
+	root := c.MkDir()
+
+	data := []byte("shim-with-untrusted-sig")
+	err := os.WriteFile(filepath.Join(d, "shim.efi"), data, 0644)
+	c.Assert(err, IsNil)
+
+	m := boot.Modeenv{
+		Mode: "run",
+		CurrentTrustedBootAssets: boot.BootAssetsMap{
+			"shim": []string{"one-hash"},
+		},
+	}
+	err = m.WriteTo("")
+	c.Assert(err, IsNil)
+
+	s.bootloaderWithTrustedAssets(map[string]string{
+		"shim.efi": "shim",
+	})
+
+	obs, _ := s.uc20UpdateObserverEncryptedSystemMockedBootloader(c)
+
+	// mock the signature check to fail
+	restore := boot.MockCheckBootAssetAgainstSignatureDB(func(assetPath string) error {
+		c.Check(assetPath, Equals, filepath.Join(d, "shim.efi"))
+		return fmt.Errorf("cannot find an secure-boot signature that is trusted by the current host's authorized signature database")
+	})
+	defer restore()
+
+	res, err := obs.Observe(gadget.ContentUpdate, gadget.SystemBoot, root, "shim.efi",
+		&gadget.ContentChange{After: filepath.Join(d, "shim.efi")})
+	c.Assert(err, ErrorMatches, `cannot use boot asset "shim": cannot find an secure-boot signature.*`)
+	c.Check(res, Equals, gadget.ChangeAbort)
+}
+
+func (s *assetsSuite) TestUpdateObserverUpdateAcceptsTrustedAsset(c *C) {
+	// Test that the observer accepts an update when the new boot asset's
+	// signature is trusted by the host's UEFI signature database.
+
+	d := c.MkDir()
+	root := c.MkDir()
+
+	data := []byte("shim-with-trusted-sig")
+	// SHA3-384
+	dataHash := "0190c0f9c71326fa3583550ba4cbfd0d1321f90f11e0d8b75113f603bb00b957bc8d1f40454cef058cec64f9df4e39f9"
+	err := os.WriteFile(filepath.Join(d, "shim.efi"), data, 0644)
+	c.Assert(err, IsNil)
+
+	m := boot.Modeenv{
+		Mode: "run",
+		CurrentTrustedBootAssets: boot.BootAssetsMap{
+			"shim": []string{"one-hash"},
+		},
+	}
+	err = m.WriteTo("")
+	c.Assert(err, IsNil)
+
+	s.bootloaderWithTrustedAssets(map[string]string{
+		"shim.efi": "shim",
+	})
+
+	obs, _ := s.uc20UpdateObserverEncryptedSystemMockedBootloader(c)
+
+	// mock the signature check to pass
+	restore := boot.MockCheckBootAssetAgainstSignatureDB(func(assetPath string) error {
+		c.Check(assetPath, Equals, filepath.Join(d, "shim.efi"))
+		return nil
+	})
+	defer restore()
+
+	res, err := obs.Observe(gadget.ContentUpdate, gadget.SystemBoot, root, "shim.efi",
+		&gadget.ContentChange{After: filepath.Join(d, "shim.efi")})
+	c.Assert(err, IsNil)
+	c.Check(res, Equals, gadget.ChangeApply)
+
+	// verify the asset was cached and tracked
+	newM, err := boot.ReadModeenv("")
+	c.Assert(err, IsNil)
+	c.Check(newM.CurrentTrustedBootAssets, DeepEquals, boot.BootAssetsMap{
+		"shim": []string{"one-hash", dataHash},
+	})
+}

boot/export_test.go

@@ -258,3 +258,11 @@ func MockCryptsetupSupportsTokenReplace(support bool) (restore func()) {
 		return support
 	})
 }
+
+func MockCheckBootAssetAgainstSignatureDB(f func(assetPath string) error) (restore func()) {
+	old := checkBootAssetAgainstSignatureDB
+	checkBootAssetAgainstSignatureDB = f
+	return func() {
+		checkBootAssetAgainstSignatureDB = old
+	}
+}

bootloader/bootloader.go

@@ -227,7 +227,7 @@ type TrustedAssetsBootloader interface {
 	DefaultCommandLine(candidate bool) (string, error)
 
 	// TrustedAssets returns a map of relative paths to asset
-	// identifers. The paths are inside the bootloader's rootdir
+	// identifiers. The paths are inside the bootloader's rootdir
 	// that are measured in the boot process. The asset
 	// identifiers correspond to the backward compatible names
 	// recorded in the modeenv (CurrentTrustedBootAssets and

bootloader/grub.go

@@ -603,7 +603,7 @@ func (g *grub) getGrubRunModeTrustedAssets() ([][]taggedPath, error) {
 }
 
 // TrustedAssets returns the map of relative paths to asset
-// identifers. The relative paths are relative to the bootloader's
+// identifiers. The relative paths are relative to the bootloader's
 // rootdir. The asset identifiers correspond to the backward
 // compatible names recorded in the modeenv (CurrentTrustedBootAssets
 // and CurrentTrustedRecoveryBootAssets).

go.mod

@@ -5,6 +5,9 @@ go 1.18
 // maze.io/x/crypto/afis imported by github.com/snapcore/secboot/tpm2
 replace maze.io/x/crypto => github.com/snapcore/maze.io-x-crypto v0.0.0-20190131090603-9b94c9afe066
 
+// local development: use local secboot with CheckImageSignatureIsValidForHost
+replace github.com/snapcore/secboot => ../secboot
+
 require (
 	github.com/bmatcuk/doublestar/v4 v4.6.1
 	github.com/canonical/go-efilib v1.7.1-0.20260310185303-7166aa858b24

go.sum

@@ -61,8 +61,6 @@ github.com/seccomp/libseccomp-golang v0.9.2-0.20220502024300-f57e1d55ea18 h1:A15
 github.com/seccomp/libseccomp-golang v0.9.2-0.20220502024300-f57e1d55ea18/go.mod h1:JA8cRccbGaA1s33RQf7Y1+q9gHmZX1yB/z9WDN1C6fg=
 github.com/snapcore/maze.io-x-crypto v0.0.0-20190131090603-9b94c9afe066 h1:InG0EmriMOiI4YgtQNOo+6fNxzLCYioo3Q3BCVLdMCE=
 github.com/snapcore/maze.io-x-crypto v0.0.0-20190131090603-9b94c9afe066/go.mod h1:VuAdaITF1MrGzxPU+8GxagM1HW2vg7QhEFEeGHbmEMU=
-github.com/snapcore/secboot v0.0.0-20260320145120-26dce572077a h1:eKljZN+SzPKM1ua7KG5E2vQdbw9JQn17tbbukHj6hvw=
-github.com/snapcore/secboot v0.0.0-20260320145120-26dce572077a/go.mod h1:+qs2Juv0XZeTmQHJgFTtAd9520h7QUWRDyvSwbJ3xEU=
 github.com/stretchr/testify v1.8.1 h1:w7B6lhMri9wdJUVmEZPGGhZzrYTPvgJArz7wNPgYKsk=
 go.etcd.io/bbolt v1.3.9 h1:8x7aARPEXiXbHmtUwAIv7eV2fQFHrLLavdiJ3uzJXoI=
 go.etcd.io/bbolt v1.3.9/go.mod h1:zaO32+Ti0PK1ivdPtgMESzuzL2VPoIG1PCQNvOdo/dE=