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

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Message-ID: <75a345d4-78ea-4434-a4cd-3f2095ce2b31@arm.com>
Date: Tue, 10 Dec 2024 10:27:56 +0100
From: Kevin Brodsky <kevin.brodsky@....com>
To: Peter Zijlstra <peterz@...radead.org>
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 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.

- Kevin