[PATCH 4/4] module, KEYS: Make use of platform keyring for signature verification

[Michal Suchánek]

On Wed, Feb 16, 2022 at 11:56:45AM +0100, Michal Suchánek wrote:
> On Tue, Feb 15, 2022 at 05:12:32PM -0500, Mimi Zohar wrote:
> > On Tue, 2022-02-15 at 21:47 +0100, Michal Suchánek wrote:
> > > Hello,
> > > 
> > > On Tue, Feb 15, 2022 at 03:08:18PM -0500, Mimi Zohar wrote:
> > > > [Cc'ing Eric Snowberg]
> > > > 
> > > > Hi Michal,
> > > > 
> > > > On Tue, 2022-02-15 at 20:39 +0100, Michal Suchanek wrote:
> > > > > Commit 278311e417be ("kexec, KEYS: Make use of platform keyring for signature verify")
> > > > > adds support for use of platform keyring in kexec verification but
> > > > > support for modules is missing.
> > > > > 
> > > > > Add support for verification of modules with keys from platform keyring
> > > > > as well.
> > > > 
> > > > Permission for loading the pre-OS keys onto the "platform" keyring and
> > > > using them is limited to verifying the kexec kernel image, nothing
> > > > else.
> > > 
> > > Why is the platform keyring limited to kexec, and nothing else?
> > > 
> > > It should either be used for everything or for nothing. You have the
> > > option to compile it in and then it should be used, and the option to
> > > not compile it in and then it cannot be used.
> > > 
> > > There are two basic use cases:
> > > 
> > > (1) there is a vendor key which is very hard to use so you sign
> > > something small and simple like shim with the vendor key, and sign your
> > > kernel and modules with your own key that's typically enrolled with shim
> > > MOK, and built into the kernel.
> > > 
> > > (2) you import your key into the firmware, and possibly disable the
> > > vendor key. You can load the kernel directly without shim, and then your
> > > signing key is typically in the platform keyring and built into the
> > > kernel.
> > > 
> > > In neither case do I see any reason to use some keyrings for kexec and
> > > other keyrings for modules.
> > 
> > When building your own kernel there isn't a problem.  Additional keys
> > may be built into the kernel image, which are loaded onto the
> > ".builtin_trusted_keys" keyring, and may be stored in MOK.  Normally
> > different keys are used for signing the kernel image and kernel
> 
> That's actually not normal.
> 
> > modules.  Kernel modules can be signed by the build time ephemeral
> > kernel module signing key, which is built into the kernel and
> > automatically loaded onto the ".builtin_trusted_keys" keyring.
> 
> Right, there is this advice to use ephemeral key to sign modules.
> 
> I don't think that's a sound advice in general. It covers only the
> special case when you build the kernel once, only rebuild the whole
> kernel and never just one module, don't use any 3rd party module, don't
> bother signing firmware (I am not sure that is supported right now but
> if you are into integrity and stuff you can see that it makes sense to
> sign it, too).
And don't forget signing ramdisk which you typically don't build only
once at kernel build time.
> 
> And you need to manage the key you use for the kernel signing, anyway.
> Sure, you could use the same ephemeral key as for the modules, enroll
> it, and shred it but then it is NOT a key different from the one you use
> for modules.
> 
> Or you could maintain a long-lived key for the kernel, but if you do I
> do NOT see any reason to not use it also for modules, in-tree and
> out-of-tree.
> 
> > Similarly distros build the kernel module signing key into the kernel,
> > which is built into the kernel and loaded onto the
> > ".builtin_trusted_keys" keyring.  By loading the pre-OS keys onto the
> > ".platform" keyring,  kexec may verify the distro or other signed
> > kernel images.
> 
> Which are signed by the same key as the modules so there is no reason to
> load the platform key at all. I don't think loading shim with kexec is
> supported.
> 
> Thanks
> 
> Michal