linux-hardening - Re: [RFC PATCH v5 00/18] pkeys-based page table hardening

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Message-ID: <d10e3bc5-5574-4498-9849-732fb373fa83@os.amperecomputing.com>
Date: Fri, 29 Aug 2025 15:31:46 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: Kevin Brodsky <kevin.brodsky@....com>, linux-hardening@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
 Andy Lutomirski <luto@...nel.org>, Catalin Marinas
 <catalin.marinas@....com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 David Hildenbrand <david@...hat.com>, Ira Weiny <ira.weiny@...el.com>,
 Jann Horn <jannh@...gle.com>, Jeff Xu <jeffxu@...omium.org>,
 Joey Gouly <joey.gouly@....com>, Kees Cook <kees@...nel.org>,
 Linus Walleij <linus.walleij@...aro.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Marc Zyngier <maz@...nel.org>,
 Mark Brown <broonie@...nel.org>, Matthew Wilcox <willy@...radead.org>,
 Maxwell Bland <mbland@...orola.com>, "Mike Rapoport (IBM)"
 <rppt@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Pierre Langlois <pierre.langlois@....com>,
 Quentin Perret <qperret@...gle.com>,
 Rick Edgecombe <rick.p.edgecombe@...el.com>,
 Ryan Roberts <ryan.roberts@....com>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>, Will Deacon <will@...nel.org>,
 linux-arm-kernel@...ts.infradead.org, linux-mm@...ck.org, x86@...nel.org
Subject: Re: [RFC PATCH v5 00/18] pkeys-based page table hardening



On 8/27/25 9:09 AM, Kevin Brodsky wrote:
> On 26/08/2025 21:18, Yang Shi wrote:
>>>>> from a contiguous cache of pages could help minimise the overhead, as
>>>>> proposed for x86 in [1].
>>>> I'm a little bit confused about how this can work. The contiguous
>>>> cache of pages should be some large page, for example, 2M. But the
>>>> page table pages allocated from the cache may have different
>>>> permissions if I understand correctly. The default permission is RO,
>>>> but some of them may become R/W at sometime, for example, when calling
>>>> set_pte_at(). You still need to split the linear mapping, right?
>>> When such a helper is called, *all* PTPs become writeable - there is no
>>> per-PTP permission switching.
>> OK, so all PTPs in the same contiguous cache will become writeable
>> even though the helper (i.e. set_pte_at()) is just called on one of
>> the PTPs.  But doesn't it compromise the page table hardening somehow?
>> The PTPs from the same cache may belong to different processes.
> First just a note that this is true regardless of how the PTPs are
> allocated (i.e. this is already the case in this version of the series).
>
> Either way, yes you are right, this approach does not introduce any
> isolation *between* page tables - pgtable helpers are able to write to
> all page tables. In principle it should be possible to use a different
> pkey for kernel and user page tables, but that would make the kpkeys
> level switching in helpers quite a bit more complicated. Isolating
> further is impractical as we have so few pkeys (just 8 on arm64).
>
> That said, what kpkeys really tries to protect against is the direct
> corruption of critical data by arbitrary (unprivileged) code. If the
> attacker is able to manipulate calls to set_pte() and the likes, kpkeys
> cannot provide much protection - even if we restricted the writes to a
> specific set of page tables, the attacker would still be able to insert
> a translation to any arbitrary physical page.

I see. Thanks for elaborating this.

Yang

>
> - Kevin