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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <939ac25a-096f-46b3-90c1-d8cd6a9e445e@os.amperecomputing.com>
Date: Tue, 26 Aug 2025 12:18:35 -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 8/25/25 12:31 AM, Kevin Brodsky wrote:
> On 21/08/2025 19:29, Yang Shi wrote:
>> Hi Kevin,
>>
>> On 8/15/25 1:54 AM, Kevin Brodsky wrote:
>>> This is a proposal to leverage protection keys (pkeys) to harden
>>> critical kernel data, by making it mostly read-only. The series includes
>>> a simple framework called "kpkeys" to manipulate pkeys for in-kernel
>>> use,
>>> as well as a page table hardening feature based on that framework,
>>> "kpkeys_hardened_pgtables". Both are implemented on arm64 as a proof of
>>> concept, but they are designed to be compatible with any architecture
>>> that supports pkeys.
>> [...]
>>
>>> Note: the performance impact of set_memory_pkey() is likely to be
>>> relatively low on arm64 because the linear mapping uses PTE-level
>>> descriptors only. This means that set_memory_pkey() simply changes the
>>> attributes of some PTE descriptors. However, some systems may be able to
>>> use higher-level descriptors in the future [5], meaning that
>>> set_memory_pkey() may have to split mappings. Allocating page tables
>> I'm supposed the page table hardening feature will be opt-in due to
>> its overhead? If so I think you can just keep kernel linear mapping
>> using PTE, just like debug page alloc.
> Indeed, I don't expect it to be turned on by default (in defconfig). If
> the overhead proves too large when block mappings are used, it seems
> reasonable to force PTE mappings when kpkeys_hardened_pgtables is enabled.
>
>>> 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.

Thanks,
Yang

>
> PTPs remain mapped RW (i.e. the base permissions set at the PTE level
> are RW). With this series, they are also all mapped with the same pkey
> (1). By default, the pkey register is configured so that pkey 1 provides
> RO access. The net result is that PTPs are RO by default, since the pkey
> restricts the effective permissions.
>
> When calling e.g. set_pte(), the pkey register is modified to enable RW
> access to pkey 1, making it possible to write to any PTP. Its value is
> restored when the function exit so that PTPs are once again RO.
>
> - Kevin