linux-kernel - Re: [RFC PATCH 12/16] arm64: mm: Map p4d/pgd with privileged pkey

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Message-ID: <20241210122355.GN8562@noisy.programming.kicks-ass.net>
Date: Tue, 10 Dec 2024 13:23:55 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Kevin Brodsky <kevin.brodsky@....com>
Cc: linux-hardening@...r.kernel.org, linux-kernel@...r.kernel.org,
	aruna.ramakrishna@...cle.com, broonie@...nel.org,
	catalin.marinas@....com, dave.hansen@...ux.intel.com,
	jannh@...gle.com, jeffxu@...omium.org, joey.gouly@....com,
	kees@...nel.org, maz@...nel.org, pierre.langlois@....com,
	qperret@...gle.com, ryan.roberts@....com, will@...nel.org,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org
Subject: Re: [RFC PATCH 12/16] arm64: mm: Map p4d/pgd with privileged pkey

On Tue, Dec 10, 2024 at 10:27:56AM +0100, Kevin Brodsky wrote:
> On 09/12/2024 11:24, Peter Zijlstra wrote:
> > On Fri, Dec 06, 2024 at 10:11:06AM +0000, Kevin Brodsky wrote:
> >> If CONFIG_KPKEYS_HARDENED_PGTABLES is enabled, map p4d/pgd pages
> >> using a privileged pkey (KPKEYS_PKEY_PGTABLES), so that they can
> >> only be written under guard(kpkeys_hardened_pgtables).
> >>
> >> The case where pgd is not page-sized is not currently handled -
> >> this is pending support for pkeys in kmem_cache.
> >>
> >> This patch is a no-op if CONFIG_KPKEYS_HARDENED_PGTABLES is disabled
> >> (default).
> > Should not this live in pagetable_*_[cd]tor() in generic code?
> 
> This would certainly be preferable but it doesn't look like such helpers
> exist for p4d/pgd. For p4d, we could potentially handle this in the
> generic __p4d_alloc(), but I'm not sure we can assume that
> p4d_alloc_one() won't be called from somewhere else. pgd_alloc() is
> entirely arch-specific so not much we can do there.

Can't we add the missing pagetable_{p4d,pgd}_[cd]tor() functions. Yes,
it will mean touching a bunch of arch code, but it shouldn't be hard.