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

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Date:   Wed, 5 May 2021 10:37:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
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 Tue, May 04, 2021 at 11:25:31PM -0700, Kees Cook wrote:

> It looks like PKS-protected page tables would be much like the
> RO-protected text pages in the sense that there is already code in
> the kernel to do things to make it writable, change text, and set it
> read-only again (alternatives, ftrace, etc).

We don't actually modify text by changing the mapping at all. We modify
through a writable (but not executable) temporary alias on the page (on
x86).

Once a mapping is RX it will *never* be writable again (until we tear it
all down).