linux-kernel - Re: [PATCH v2 3/4] mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM

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Date:   Mon, 15 May 2023 12:36:00 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Ruihan Li <lrh2000@....edu.cn>
Cc:     linux-mm@...ck.org, linux-usb@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH v2 3/4] mm: page_table_check: Make it dependent on EXCLUSIVE_SYSTEM_RAM

On Mon, May 15, 2023 at 9:10 AM Ruihan Li <lrh2000@....edu.cn> wrote:
>
> Without EXCLUSIVE_SYSTEM_RAM, users are allowed to map arbitrary
> physical memory regions into the userspace via /dev/mem. At the same
> time, pages may change their properties (e.g., from anonymous pages to
> named pages) while they are still being mapped in the userspace, leading
> to "corruption" detected by the page table check.
>
> To avoid these false positives, this patch makes PAGE_TABLE_CHECK
> depends on EXCLUSIVE_SYSTEM_RAM. This dependency is understandable
> because PAGE_TABLE_CHECK is a hardening technique but /dev/mem without
> STRICT_DEVMEM (i.e., !EXCLUSIVE_SYSTEM_RAM) is itself a security
> problem.
>
> Even with EXCLUSIVE_SYSTEM_RAM, I/O pages may be still allowed to be
> mapped via /dev/mem. However, these pages are always considered as named
> pages, so they won't break the logic used in the page table check.
>
> Cc: <stable@...r.kernel.org> # 5.17
> Signed-off-by: Ruihan Li <lrh2000@....edu.cn>

Acked-by: Pasha Tatashin <pasha.tatashin@...een.com>

Thank you,
Pasha