pkg/build: consistently chown new files

Kernel build runs under sandboxed user, while the rest of the code
runs under root. Sandboxed user does not have access to root files.
Chown all new created files for sandboxed user.
Currently builds fail with:

unable to write 'random state'
writing new private key to 'certs/signing_key.pem'
certs/signing_key.pem: Permission denied
22979106625176:error:0200100D:system library:fopen:Permission
	denied:bss_file.c:398:fopen('certs/signing_key.pem','w')
22979106625176:error:20074002:BIO routines:FILE_CTRL:system lib:bss_file.c:400:
certs/Makefile:55: recipe for target 'certs/signing_key.pem' failed
make[1]: *** [certs/signing_key.pem] Error 1
Makefile:1053: recipe for target 'certs' failed
make: *** [certs] Error 2

1 files changed, 36 insertions, 36 deletions

diff --git a/pkg/build/linux.go b/pkg/build/linux.go
index 541929675..36f18b84f 100644
--- a/pkg/build/linux.go
+++ b/pkg/build/linux.go
@@ -24,30 +24,6 @@ type linux struct{}
 
 var _ signer = linux{}
 
-// Key for module signing.
-const moduleSigningKey = `-----BEGIN PRIVATE KEY-----
-MIIBVAIBADANBgkqhkiG9w0BAQEFAASCAT4wggE6AgEAAkEAxu5GRXw7d13xTLlZ
-GT1y63U4Firk3WjXapTgf9radlfzpqheFr5HWO8f11U/euZQWXDzi+Bsq+6s/2lJ
-AU9XWQIDAQABAkB24ZxTGBv9iMGURUvOvp83wRRkgvvEqUva4N+M6MAXagav3GRi
-K/gl3htzQVe+PLGDfbIkstPJUvI2izL8ZWmBAiEA/P72IitEYE4NQj4dPcYglEYT
-Hbh2ydGYFbYxvG19DTECIQDJSvg7NdAaZNd9faE5UIAcLF35k988m9hSqBjtz0tC
-qQIgGOJC901mJkrHBxLw8ViBb9QMoUm5dVRGLyyCa9QhDqECIQCQGLX4lP5DVrsY
-X43BnMoI4Q3o8x1Uou/JxAIMg1+J+QIgamNCPBLeP8Ce38HtPcm8BXmhPKkpCXdn
-uUf4bYtfSSw=
------END PRIVATE KEY-----
------BEGIN CERTIFICATE-----
-MIIBvzCCAWmgAwIBAgIUKoM7Idv4nw571nWDgYFpw6I29u0wDQYJKoZIhvcNAQEF
-BQAwLjEsMCoGA1UEAwwjQnVpbGQgdGltZSBhdXRvZ2VuZXJhdGVkIGtlcm5lbCBr
-ZXkwIBcNMjAxMDA4MTAzMzIwWhgPMjEyMDA5MTQxMDMzMjBaMC4xLDAqBgNVBAMM
-I0J1aWxkIHRpbWUgYXV0b2dlbmVyYXRlZCBrZXJuZWwga2V5MFwwDQYJKoZIhvcN
-AQEBBQADSwAwSAJBAMbuRkV8O3dd8Uy5WRk9cut1OBYq5N1o12qU4H/a2nZX86ao
-Xha+R1jvH9dVP3rmUFlw84vgbKvurP9pSQFPV1kCAwEAAaNdMFswDAYDVR0TAQH/
-BAIwADALBgNVHQ8EBAMCB4AwHQYDVR0OBBYEFPhQx4etmYw5auCJwIO5QP8Kmrt3
-MB8GA1UdIwQYMBaAFPhQx4etmYw5auCJwIO5QP8Kmrt3MA0GCSqGSIb3DQEBBQUA
-A0EAK5moCH39eLLn98pBzSm3MXrHpLtOWuu2p696fg/ZjiUmRSdHK3yoRONxMHLJ
-1nL9cAjWPantqCm5eoyhj7V7gg==
------END CERTIFICATE-----`
-
 func (linux linux) build(params *Params) error {
 	if err := linux.buildKernel(params); err != nil {
 		return err
@@ -62,14 +38,11 @@ func (linux linux) sign(params *Params) (string, error) {
 	return elfBinarySignature(filepath.Join(params.OutputDir, "obj", "vmlinux"))
 }
 
-func (linux) buildKernel(params *Params) error {
+func (linux linux) buildKernel(params *Params) error {
 	configFile := filepath.Join(params.KernelDir, ".config")
-	if err := osutil.WriteFile(configFile, params.Config); err != nil {
+	if err := linux.writeFile(configFile, params.Config); err != nil {
 		return fmt.Errorf("failed to write config file: %v", err)
 	}
-	if err := osutil.SandboxChown(configFile); err != nil {
-		return err
-	}
 	// One would expect olddefconfig here, but olddefconfig is not present in v3.6 and below.
 	// oldconfig is the same as olddefconfig if stdin is not set.
 	// Note: passing in compiler is important since 4.17 (at the very least it's noted in the config).
@@ -95,13 +68,10 @@ func (linux) buildKernel(params *Params) error {
 		ccParam = params.Ccache + " " + ccParam
 		// Ensure CONFIG_GCC_PLUGIN_RANDSTRUCT doesn't prevent ccache usage.
 		// See /Documentation/kbuild/reproducible-builds.rst.
+		const seed = `const char *randstruct_seed = "e9db0ca5181da2eedb76eba144df7aba4b7f9359040ee58409765f2bdc4cb3b8";`
 		gccPluginsDir := filepath.Join(params.KernelDir, "scripts", "gcc-plugins")
 		if osutil.IsExist(gccPluginsDir) {
-			err := osutil.WriteFile(filepath.Join(gccPluginsDir,
-				"randomize_layout_seed.h"),
-				[]byte("const char *randstruct_seed = "+
-					"\"e9db0ca5181da2eedb76eba144df7aba4b7f9359040ee58409765f2bdc4cb3b8\";"))
-			if err != nil {
+			if err := linux.writeFile(filepath.Join(gccPluginsDir, "randomize_layout_seed.h"), []byte(seed)); err != nil {
 				return err
 			}
 		}
@@ -112,8 +82,7 @@ func (linux) buildKernel(params *Params) error {
 	// calculation.
 	certsDir := filepath.Join(params.KernelDir, "certs")
 	if osutil.IsExist(certsDir) {
-		err := osutil.WriteFile(filepath.Join(certsDir, "signing_key.pem"), []byte(moduleSigningKey))
-		if err != nil {
+		if err := linux.writeFile(filepath.Join(certsDir, "signing_key.pem"), []byte(moduleSigningKey)); err != nil {
 			return err
 		}
 	}
@@ -179,6 +148,13 @@ func (linux) clean(kernelDir, targetArch string) error {
 	return runMake(kernelDir, "distclean")
 }
 
+func (linux) writeFile(file string, data []byte) error {
+	if err := osutil.WriteFile(file, data); err != nil {
+		return err
+	}
+	return osutil.SandboxChown(file)
+}
+
 func runMake(kernelDir string, args ...string) error {
 	args = append(args, fmt.Sprintf("-j%v", runtime.NumCPU()))
 	cmd := osutil.Command("make", args...)
@@ -228,3 +204,27 @@ func elfBinarySignature(bin string) (string, error) {
 	}
 	return hex.EncodeToString(hasher.Sum(nil)), nil
 }
+
+// moduleSigningKey is a constant module signing key for reproducible builds.
+const moduleSigningKey = `-----BEGIN PRIVATE KEY-----
+MIIBVAIBADANBgkqhkiG9w0BAQEFAASCAT4wggE6AgEAAkEAxu5GRXw7d13xTLlZ
+GT1y63U4Firk3WjXapTgf9radlfzpqheFr5HWO8f11U/euZQWXDzi+Bsq+6s/2lJ
+AU9XWQIDAQABAkB24ZxTGBv9iMGURUvOvp83wRRkgvvEqUva4N+M6MAXagav3GRi
+K/gl3htzQVe+PLGDfbIkstPJUvI2izL8ZWmBAiEA/P72IitEYE4NQj4dPcYglEYT
+Hbh2ydGYFbYxvG19DTECIQDJSvg7NdAaZNd9faE5UIAcLF35k988m9hSqBjtz0tC
+qQIgGOJC901mJkrHBxLw8ViBb9QMoUm5dVRGLyyCa9QhDqECIQCQGLX4lP5DVrsY
+X43BnMoI4Q3o8x1Uou/JxAIMg1+J+QIgamNCPBLeP8Ce38HtPcm8BXmhPKkpCXdn
+uUf4bYtfSSw=
+-----END PRIVATE KEY-----
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----`