linux-kernel - Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate

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: <1e8ca159-bf4a-47ab-b965-c7e30ad51b28@lucifer.local>
Date: Mon, 11 Aug 2025 10:10:58 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Harry Yoo <harry.yoo@...cle.com>
Cc: Dennis Zhou <dennis@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Ryabinin <ryabinin.a.a@...il.com>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
        Tejun Heo <tj@...nel.org>, Uladzislau Rezki <urezki@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Christoph Lameter <cl@...two.org>,
        David Hildenbrand <david@...hat.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        "H. Peter Anvin" <hpa@...or.com>, kasan-dev@...glegroups.com,
        Mike Rapoport <rppt@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
        linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>, Thomas Huth <thuth@...hat.com>,
        John Hubbard <jhubbard@...dia.com>, Michal Hocko <mhocko@...e.com>,
        "Liam R. Howlett" <Liam.Howlett@...cle.com>, linux-mm@...ck.org,
        "Kirill A. Shutemov" <kas@...nel.org>,
        Oscar Salvador <osalvador@...e.de>, Jane Chu <jane.chu@...cle.com>,
        Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>,
        "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, Alistair Popple <apopple@...dia.com>,
        Joao Martins <joao.m.martins@...cle.com>, linux-arch@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use
 {pgd,p4d}_populate_kernel()

On Mon, Aug 11, 2025 at 02:34:19PM +0900, Harry Yoo wrote:
> Introduce and use {pgd,p4d}_populate_kernel() in core MM code when
> populating PGD and P4D entries for the kernel address space.
> These helpers ensure proper synchronization of page tables when
> updating the kernel portion of top-level page tables.
>
> Until now, the kernel has relied on each architecture to handle
> synchronization of top-level page tables in an ad-hoc manner.
> For example, see commit 9b861528a801 ("x86-64, mem: Update all PGDs for
> direct mapping and vmemmap mapping changes").
>
> However, this approach has proven fragile for following reasons:
>
>   1) It is easy to forget to perform the necessary page table
>      synchronization when introducing new changes.
>      For instance, commit 4917f55b4ef9 ("mm/sparse-vmemmap: improve memory
>      savings for compound devmaps") overlooked the need to synchronize
>      page tables for the vmemmap area.
>
>   2) It is also easy to overlook that the vmemmap and direct mapping areas
>      must not be accessed before explicit page table synchronization.
>      For example, commit 8d400913c231 ("x86/vmemmap: handle unpopulated
>      sub-pmd ranges")) caused crashes by accessing the vmemmap area
>      before calling sync_global_pgds().
>
> To address this, as suggested by Dave Hansen, introduce _kernel() variants
> of the page table population helpers, which invoke architecture-specific
> hooks to properly synchronize page tables. These are introduced in a new
> header file, include/linux/pgalloc.h, so they can be called from common code.
>
> They reuse existing infrastructure for vmalloc and ioremap.
> Synchronization requirements are determined by ARCH_PAGE_TABLE_SYNC_MASK,
> and the actual synchronization is performed by arch_sync_kernel_mappings().
>
> This change currently targets only x86_64, so only PGD and P4D level
> helpers are introduced. In theory, PUD and PMD level helpers can be added
> later if needed by other architectures.
>
> Currently this is a no-op, since no architecture sets
> PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
>
> Cc: <stable@...r.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
> ---
>  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++

Could we put this in the correct place in MAINTAINERS please? I think
MEMORY MANAGEMENT - CORE is correct, given the below file is there.

>  include/linux/pgtable.h |  4 ++--

Thanks!