linux-kernel - Re: [PATCH v3 7/7] firmware: coreboot: Request table region for exclusive access

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Message-ID: <153385579866.220756.16086660810932774163@swboyd.mtv.corp.google.com>
Date:   Thu, 09 Aug 2018 16:03:18 -0700
From:   Stephen Boyd <swboyd@...omium.org>
To:     Julius Werner <jwerner@...omium.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Wei-Ning Huang <wnhuang@...omium.org>,
        Julius Werner <jwerner@...omium.org>,
        Brian Norris <briannorris@...omium.org>, samuel@...lland.org
Subject: Re: [PATCH v3 7/7] firmware: coreboot: Request table region for exclusive
 access

Quoting Julius Werner (2018-08-09 14:07:31)
> On Thu, Aug 9, 2018 at 10:17 AM Stephen Boyd <swboyd@...omium.org> wrote:
> >
> > Call request_mem_region() on the entire coreboot table to make sure
> > other devices don't attempt to map the coreboot table in their drivers.
> > If drivers need that support, it would be better to provide bus APIs
> > they can use to do that through the mapping created in this file.
> >
> 
> Does this prevent userspace from mapping this region via /dev/mem? If
> so, let's please not do it to not break compatibility with existing
> tools.

No it doesn't break. I can still read the memory here with /dev/mem (and
cbmem). But that seems to be because of a couple reasons. First, I have
CONFIG_STRICT_DEVMEM=y but CONFIG_IO_STRICT_DEVMEM=n. This allows me to
map memory that isn't system ram. If CONFIG_IO_STRICT_DEVMEM=y then
mapping this I/O region through /dev/mem here would be denied .
Furthermore, I see that my system RAM excludes this coreboot table so it
doesn't fall into the bucket that CONFIG_STRICT_DEVMEM would find.

> (I guess an alternative would be to rewrite 'cbmem' to use
> /sys/bus/coreboot/devices if available to get its coreboot table
> information. But we'd still need to maintain the old path for
> backwards compatibility anyway, so that would really just make it more
> complicated.)

This sounds like a good idea. Userspace reaching into /dev/mem is not
good from a kernel hardening perspective. That's why those strict devmem
configs exist. Can cbmem be updated to query information from device
drivers instead, so that we can enable CONFIG_IO_STRICT_DEVMEM as well?