Re: [RFC PATCH v2 17/19] heki: x86: Update permissions counters during text patching

[Peter Zijlstra]

On Wed, Nov 29, 2023 at 03:07:15PM -0600, Madhavan T. Venkataraman wrote:

> Kernel Lockdown
> ---------------
> 
> But, we must provide at least some security in V2. Otherwise, it is useless.
> 
> So, we have implemented what we call a kernel lockdown. At the end of kernel
> boot, Heki establishes permissions in the extended page table as mentioned
> before. Also, it adds an immutable attribute for kernel text and kernel RO data.
> Beyond that point, guest requests that attempt to modify permissions on any of
> the immutable pages will be denied.
> 
> This means that features like FTrace and KProbes will not work on kernel text
> in V2. This is a temporary limitation. Once authentication is in place, the
> limitation will go away.

So either you're saying your patch 17 / text_poke is broken (so why
include it ?!?) or your statement above is incorrect. Pick one.


> __text_poke()
> 	This function is called by various features to patch text.
> 	This calls heki_text_poke_start() and heki_text_poke_end().
> 
> 	heki_text_poke_start() is called to add write permissions to the
> 	extended page table so that text can be patched. heki_text_poke_end()
> 	is called to revert write permissions in the extended page table.

This, if text_poke works, then static_call / jump_label / ftrace and
everything else should work, they all rely on this.


> Peter mentioned the following:
> 
> "if you want to mirror the native PTEs why don't you hook into the
> paravirt page-table muck and get all that for free?"
> 
> We did consider using a shadow page table kind of approach so that guest page table
> modifications can be intercepted and reflected in the page table entry. We did not
> do this for two reasons:
> 
> - there are bits in the page table entry that are not permission bits. We would like
>   the guest kernel to be able to modify them directly.

This statement makes no sense.

> - we cannot tell a genuine request from an attack.

Why not? How is an explicit call different from an explicit call in a
paravirt hook?

>From a maintenance pov we already hate paravirt with a passion, but it
is ever so much better than sprinkling yet another pile of crap
(heki_*) around.