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

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Date:   Wed, 5 May 2021 19:51:08 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "keescook@...omium.org" <keescook@...omium.org>
CC:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "rppt@...nel.org" <rppt@...nel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "Weiny, Ira" <ira.weiny@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>
Subject: Re: [PATCH RFC 0/9] PKS write protected page tables

On Tue, 2021-05-04 at 23:25 -0700, Kees Cook wrote:
> > infrastructure follow-on’s are planned to enable keys to be set to
> > the same 
> > permissions globally. Since this usage needs a key to be set globally
> > read-only by default, a small temporary solution is hacked up in
> > patch 8. Long 
> > term, PKS protected page tables would use a better and more generic
> > solution 
> > to achieve this.
> > 
> > [1]
> > https://lore.kernel.org/lkml/20210401225833.566238-1-ira.weiny@intel.com/
> 
> Ah, neat!
> 
> > [2]
> > https://lore.kernel.org/lkml/20210405203711.1095940-1-rick.p.edgecombe@intel.com/
> 
> Ooh. What does this do for performance? It sounds like less TLB
> pressure, IIUC?

Yea, less TLB pressure, faster page table walks in theory. There was
some testing that showed having all 4k pages was bad for performance:
https://lore.kernel.org/linux-mm/213b4567-46ce-f116-9cdf-bbd0c884eb3c@linux.intel.com/

I'm not sure exactly how much breakage is needed before problems start
to show up, but there was also someone posting that large amounts of
tracing was noticeable for their workload.