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From:   Dan Williams <dan.j.williams@intel.com>
Date:   Thu, 7 Apr 2022 16:43:10 -0700
Message-ID: <CAPcyv4hFxVAKgUqt-mua5_1NZkfyKKFL-3F0tSWuSmD=4Z1sBw@mail.gmail.com>
Subject: Re: [PATCH v4] /dev/mem: Revoke mappings when a driver claims the region
To:     Kees Cook <keescook@chromium.org>
Cc:     Greg KH <gregkh@linuxfoundation.org>,
        Arnd Bergmann <arnd@arndb.de>, Ingo Molnar <mingo@redhat.com>,
        Matthew Wilcox <willy@infradead.org>,
        Russell King <linux@arm.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>
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On Thu, Apr 7, 2022 at 11:47 AM Dan Williams <dan.j.williams@intel.com> wrote:
>
> On Wed, Apr 6, 2022 at 12:46 PM Kees Cook <keescook@chromium.org> wrote:
> >
> > *thread necromancy*
>
> It's alive!
>
> >
> > Hi Dan,
> >
> > I'm doing a KSPP bug scrub and am reviewing
> > https://github.com/KSPP/linux/issues/74 again.
> >
> > Do you have a chance to look at this? I'd love a way to make mmap()
> > behave the same way as read() for the first meg of /dev/mem.
>
> You want 0-reads or SIGBUS when attempting to access the first 1MB?
>
> Because it sounds like what you want is instead of loudly failing with
> -EPERM in drivers/char/mem.c::mmap_mem() you want it to silently
> succeed but swap in the zero page, right? Otherwise if it's SIGBUS
> then IO_STRICT_DEVMEM=y + marking that span as IORESOURCE_BUSY will
> "Do the Right Thing (TM).".

In other words, if IO_STRICT_DEVMEM is enabled then the enforcement is
already there at least for anything marked IORESOURCE_BUSY. So if
tools are ok with that protection today, maybe there is no need to do
the zero page dance. I.e. legacy tools the read(2) /dev/mem below 1MB
get zeroes, and apparently no tools were mmap'ing below 1MB otherwise
they would have complained by now? At least Fedora is shipping
IO_STRICT_DEVMEM these days:

https://src.fedoraproject.org/rpms/kernel/blob/rawhide/f/kernel-x86_64-fedora.config#_2799