Date: Wed, 5 May 2021 11:38:39 -0700 From: Kees Cook <keescook@...omium.org> To: Peter Zijlstra <peterz@...radead.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 Wed, May 05, 2021 at 10:37:29AM +0200, Peter Zijlstra wrote: > 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). Yes, quite true. I was trying to answer the concern about "is it okay that there is a routine in the kernel that can write to page tables (via temporary disabling of PKS)?" by saying "yes, this is fine -- we already have similar routines in the kernel that bypass memory protections, and that's okay because the defense is primarily about blocking flaws that allow attacker-controlled writes to be used to leverage greater control over kernel state, of which the page tables are pretty central. :) -- Kees Cook
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