Re: [PATCH] MODSIGN: Change default key details [ver #2]

[Abelardo Ricart III <aricart@memnix.com>]

On Sun, 2015-05-03 at 22:16 -0700, Linus Torvalds wrote:
> On May 3, 2015 21:42, "Abelardo Ricart III" <aricart@memnix.com> wrote:
> >
> > That's correct. I was under the impression that having the Makefile generate
> > the signing keys was something that was done just to prevent a build failure
> > with CONFIG_MODULE_SIG but no keys.
> No, that's absolutely not the case.
> In fact, the whole "external keys" model is entirely bogus for any same use 
> case.
> The sane use case is to have the build process generate a random key at build 
> time, that gets thrown away after installing the kernel and modules. That, 
> together with "require signed modules" makes module as safe as building 
> everything into the kernel - you won't be open to things like rootkits that 
> try to load modules, because nobody has access to the key any more.
> The only time you will have an external non-generated key is when you either 
> want to do the insane secure boot thing, or when a distro builds an official 
> kernel. But those are *not* the common development situations.
> So the "generated random throwaway key" is absolutely not some of special 
> case to not break the build. It should be seen as the *default* case.
>    Linus

Here's a (barely tested) patch to show what I mean with the config option. The
default case is to always generate a new key at build (MODULE_SIG_BUILDGEN=y)
and fallback on generating keys during build only if one doesn't exist
(MODULE_SIG_BUILDGEN=n). 

This fixes the issues with keys being unexpectedly overwritten when you don't
want them to be. Also fixes keys _not_ always being regenerated when they
really should be (the default use case).

---
diff --git a/init/Kconfig b/init/Kconfig
index dc24dec..5ab8b97 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1903,6 +1903,16 @@ config MODULE_SIG_ALL
 comment "Do not forget to sign required modules with scripts/sign-file"
        depends on MODULE_SIG_FORCE && !MODULE_SIG_ALL
 
+config MODULE_SIG_BUILDGEN
+       bool "Always generate keys during build"
+       default y
+       depends on MODULE_SIG
+       help
+         Always generate new signing keys at build time. Only say N here if
+         you intend on supplying your own signing keys.
+
+         Say Y here unless you know what you are doing.
+
 choice
        prompt "Which hash algorithm should modules be signed with?"
        depends on MODULE_SIG
diff --git a/kernel/Makefile b/kernel/Makefile
index 60c302c..86d836d 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -170,6 +170,15 @@ ifndef CONFIG_MODULE_SIG_HASH
 $(error Could not determine digest type to use from kernel config)
 endif
 
+.PHONY: generate_keys
+ifeq ($(CONFIG_MODULE_SIG_BUILDGEN),y)
+       # Always generate new signing keys
+       generate_keys: signing_key.priv signing_key.x509 FORCE
+else
+       # Only generate signing keys if they don't exist
+       generate_keys: | signing_key.priv signing_key.x509
+endif
+
 signing_key.priv signing_key.x509: x509.genkey
        @echo "###"
        @echo "### Now generating an X.509 key pair to be used for signing modules."