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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 17 Jun 2020 10:37:52 +0200
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Subject: Re: [PATCH] acpi: disallow loading configfs acpi tables when locked down
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Len Brown <lenb@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        LKML <linux-kernel@vger.kernel.org>,
        ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>
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On Wed, 17 Jun 2020 at 00:21, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
>
> Hi Rafael, Len,
>
> Looks like I should have CC'd you on this patch. This is probably
> something we should get into 5.8-rc2, so that it can then get put into
> stable kernels, as some people think this is security sensitive.
> Bigger picture is this:
>
> https://data.zx2c4.com/american-unsigned-language-2.gif
> https://data.zx2c4.com/american-unsigned-language-2-fedora-5.8.png
>
> Also, somebody mentioned to me that Microsoft's ACPI implementation
> disallows writes to system memory as a security mitigation. I haven't
> looked at what that actually entails, but I wonder if entirely
> disabling support for ACPI_ADR_SPACE_SYSTEM_MEMORY would be sensible.
> I haven't looked at too many DSDTs. Would that break real hardware, or
> does nobody do that? Alternatively, the range of acceptable addresses
> for SystemMemory could exclude kernel memory. Would that break
> anything? Have you heard about Microsoft's mitigation to know more
> details on what they figured out they could safely restrict without
> breaking hardware? Either way, food for thought I suppose.
>

ACPI_ADR_SPACE_SYSTEM_MEMORY may be used for everything that is memory
mapped, i.e., PCIe ECAM space, GPIO control registers etc.

I agree that using ACPI_ADR_SPACE_SYSTEM_MEMORY for any memory that is
under the kernel's control is a bad idea, and this should be easy to
filter out: the SystemMemory address space handler needs the ACPI
support routines to map the physical addresses used by AML into
virtual kernel addresses, so all these accesses go through
acpi_os_ioremap(). So as a first step, it should be reasonable to put
a lockdown check there, and fail any access to OS owned memory if
lockdown is enabled, and print a warning if it is not.