Re: [PATCH v1 3/9] virt: Implement Heki common code

From: Mickaël Salaün
Date: Mon May 29 2023 - 12:03:49 EST

On 17/05/2023 14:47, Madhavan T. Venkataraman wrote:
Sorry for the delay. See inline...

On 5/8/23 12:29, Wei Liu wrote:
On Fri, May 05, 2023 at 05:20:40PM +0200, Mickaël Salaün wrote:
From: Madhavan T. Venkataraman <madvenka@xxxxxxxxxxxxxxxxxxx>

Hypervisor Enforced Kernel Integrity (Heki) is a feature that will use
the hypervisor to enhance guest virtual machine security.


Define the config variables for the feature. This feature depends on
support from the architecture as well as the hypervisor.

Enabling HEKI

Define a kernel command line parameter "heki" to turn the feature on or
off. By default, Heki is on.

For such a newfangled feature can we have it off by default? Especially
when there are unsolved issues around dynamically loaded code.

Yes. We can certainly do that.

By default the Kconfig option should definitely be off. We also need to change the Kconfig option to only be set if kernel module, JIT, kprobes and other dynamic text change feature are disabled at build time (see discussion with Sean).

With this new Kconfig option for the static case, I think the boot option should be on by default because otherwise it would not really be possible to switch back to on later without taking the risk to silently breaking users' machines. However, we should rename this option to something like "heki_static" to be in line with the new Kconfig option.

The goal of Heki is to improve and complement kernel self-protection mechanisms (which don't have boot time options), and to make it available to everyone, see
In practice, it would then be kind of useless to be required to set a boot option to enable Heki (rather than to disable it).

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 3604074a878b..5cf5a7a97811 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -297,6 +297,7 @@ config X86
+ select ARCH_SUPPORTS_HEKI if X86_64

Why is there a restriction on X86_64?

We want to get the PoC working and reviewed on X64 first. We have tested this only on X64 so far.

X86_64 includes Intel CPUs, which can support EPT and MBEC, which are a requirement for Heki. ARM might have similar features but we're focused on x86 for now.

As a side note, I only have access to an Intel machine, which means that I cannot work on AMD support. However, I'll be pleased to implement such support if I get access to a machine with a recent AMD CPU.

def_bool y
diff --git a/arch/x86/include/asm/sections.h b/arch/x86/include/asm/sections.h
index a6e8373a5170..42ef1e33b8a5 100644
--- a/arch/x86/include/asm/sections.h
+++ b/arch/x86/include/asm/sections.h
+ * Gather all of the statically defined sections so heki_late_init() can
+ * protect these sections in the host page table.
+ *
+ * The sections are defined under "SECTIONS" in
+ * Keep this array in sync with SECTIONS.
+ */

This seems a bit fragile, because it requires constant attention from
people who care about this functionality. Can this table be
automatically generated?

We realize that. But I don't know of a way this can be automatically generated. Also, the permissions for
each section is specific to the use of that section. The developer who introduces a new section is the
one who will know what the permissions should be.

If any one has any ideas of how we can generate this table automatically or even just add a build time check
of some sort, please let us know.

One clean solution might be to parse the file, extract section and their permission, and fill that into an automatically generated header file.

Another way to do it would be to extract sections and associated permissions with objdump, but that could be an issue because of longer build time.

A better solution would be to extract such sections and associated permissions at boot time. I guess the kernel already has such helpers used in early boot.