linux-hardening - Re: [PATCH RFC 0/9] PKS write protected page tables

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Date:   Wed, 5 May 2021 13:56:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Rick Edgecombe <rick.p.edgecombe@...el.com>, dave.hansen@...el.com,
        luto@...nel.org, linux-mm@...ck.org, x86@...nel.org,
        akpm@...ux-foundation.org, linux-hardening@...r.kernel.org,
        kernel-hardening@...ts.openwall.com, ira.weiny@...el.com,
        rppt@...nel.org, dan.j.williams@...el.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/9] PKS write protected page tables

On Wed, May 05, 2021 at 01:08:35PM +0200, Vlastimil Babka wrote:
> On 5/5/21 2:30 AM, Rick Edgecombe wrote:

> > Why use PKS for this?
> > =====================
> > PKS is an upcoming CPU feature that allows supervisor virtual memory 
> > permissions to be changed without flushing the TLB, like PKU does for user 
> > memory. Protecting page tables would normally be really expensive because you 
> > would have to do it with paging itself. PKS helps by providing a way to toggle 
> > the writability of the page tables with just a per-cpu MSR.
> 
> I can see in patch 8/9 that you are flipping the MSR around individual
> operations on page table entries. In my patch I hooked making the page table
> writable to obtaining the page table lock (IIRC I had only the PTE level fully
> handled though). Wonder if that would be better tradeoff even for your MSR approach?

There's also the HIGHPTE code we could abuse to kmap an alias while
we're at it.