Re: [PATCH] acpi: disallow loading configfs acpi tables when locked down

[Jason A. Donenfeld]

On Wed, Jun 17, 2020 at 2:38 AM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Wed, 17 Jun 2020 at 00:21, Jason A. Donenfeld <Jason@xxxxxxxxx> 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.

Makes sense. Though I was thinking of doing this unconditionally, not
just for lockdown, as a "realer" security mitigation (hence CCing
kernel-hardening), against bugs that manage to mess with acpi things.
A second mitigation might be marking inmemory acpi bytecode pages as
read only.