Re: [PATCH v1 0/4] ima: require signed user-space initialization

From: Andrew Morton
Date: Tue Jul 15 2014 - 17:33:27 EST

On Tue, 15 Jul 2014 15:54:19 +0300 Dmitry Kasatkin <d.kasatkin@xxxxxxxxxxx> wrote:

> Currently secure IMA/EVM initialization has to be done from the initramfs,
> embedded in the signed kernel image. Many systems do not want to use
> initramfs or usage of embedded initramfs makes it difficult to have
> multi-target kernels.
> This is a very simple patchset which makes it possible to perform secure
> initialization by requiring initial user-space to be signed.
> It does it by:
> - introducing IMA public keys loading hook
> - loading IMA trusted public key into .ima trusted keyring
> - making default IMA appraisal policy to require everything to be signed
> When builtin initramfs is not in use, keys cannot be read from initcalls,
> because root filesystem is not yet mounted. In order to read keys before
> executing init process, ima_prepare_keys() hook is introduced. Reading
> public keys from the kernel is justified because signature verification
> key is needed in order to verify anything else which is read from the
> file system. Public keys are X509 certificates and itself signed by the
> trusted key from the .system keyring. Kernel BIG KEYS support is an example
> of reading keys directly by the kernel.
> CONFIG_IMA_APPRAISE_SIGNED_INIT kernel option is provided to make the IMA
> default appraisal policy to required signature validation. Signed init
> process need to initialize EVM key and load appropriate IMA policy which
> would not require everything to be signed.
> Unless real '/sbin/init' is signed, a simple and practical way is to place
> all signed programs, libraries, scripts and configuration files under
> dedicated directory, for example '/ima', and run signed init process by
> providing a kernel command line parameter 'init=/ima/init'

The kernel may already have loaded kernel modules before it gets around
to mounting rootfs and running /sbin/init. How does that fit into the
overall signing scheme? And how did /sbin/modprobe get its signature

The proposed interface and implementation look reasonable to me.
Opening and reading a file from the root fs is a bit unusual, but we
already do something similar with "/sbin/init" and the reasoning here
is similar.

The only alternative I can immediately think of is to bundle the public
keys into a kernel module and load them into the kernel that way but

- if/when we implement module signing, we have a chicken-and-egg problem

- doing it via a kernel module seems a bit fake - a bit of trickery
to reduce code duplication. Better to do it explicitly.

One thing I'm wondering: integrity_read_file() is a very simple
open-file-and-slurp-it-into-memory. Have you checked whether other
code sites are doing the same thing? Perhaps integrity_read_file()
should be in ./lib/ and other callsites can be converted to share it?

That comment in integrity_read_file() is completely useless :(

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