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Date:   Wed, 5 May 2021 11:38:39 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Rick Edgecombe <rick.p.edgecombe@intel.com>, dave.hansen@intel.com,
        luto@kernel.org, linux-mm@kvack.org, x86@kernel.org,
        akpm@linux-foundation.org, linux-hardening@vger.kernel.org,
        kernel-hardening@lists.openwall.com, ira.weiny@intel.com,
        rppt@kernel.org, dan.j.williams@intel.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH RFC 0/9] PKS write protected page tables
Message-ID: <202105051132.7958C3B@keescook>
References: <20210505003032.489164-1-rick.p.edgecombe@intel.com>
 <202105042253.ECBBF6B6@keescook>
 <YJJZSdVoP6yBbIjN@hirez.programming.kicks-ass.net>
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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