linux-kernel - AMD / Memory Protection Keys

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Date:   Wed, 29 Apr 2020 12:18:06 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        jon.grimm@....com,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Lendacky <Thomas.Lendacky@....com>
Subject: AMD / Memory Protection Keys

The docs for AMD's implementation of protection keys showed up[1].
Welcome to the party!

It all looks fine, except for this nugget:

	The MPK mechanism is ignored in the following cases:
	... for pages marked in the paging structures as read-only
	(R/W=0) or as supervisor addresses (U/S=0)

That R/W=0 would mean that you can't access-disable read-only pages,
which seems a bit goofy.  It's certainly a feature that I could imagine
folks wanting to have.  Read-only pages might happen in unexpected
places for things like mmap(PROT_WRITE)'d files or even CoW pages after
fork().

Is this an error in the docs?

1. https://www.amd.com/system/files/TechDocs/24593.pdf