linux-kernel - Re: [PATCH] arm64: Add translation functions for /dev/mem read/write

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Date:   Fri, 5 May 2017 15:55:06 +0100
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     "Goel, Sameer" <sgoel@...eaurora.org>,
        Leif Lindholm <leif.lindholm@...aro.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Shanker Donthineni <shankerd@...eaurora.org>,
        Mark Rutland <mark.rutland@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH] arm64: Add translation functions for /dev/mem read/write

On Thu, May 04, 2017 at 07:40:50AM +0100, Ard Biesheuvel wrote:
> On 3 May 2017 at 22:47, Goel, Sameer <sgoel@...eaurora.org> wrote:
> >
> >
> > On 5/3/2017 2:18 PM, Leif Lindholm wrote:
> >> On Wed, May 03, 2017 at 11:07:45AM -0600, Goel, Sameer wrote:
> >>> On 5/3/2017 5:26 AM, Will Deacon wrote:
> >>>> [adding some /dev/mem fans to cc]
> >>>>
> >>>> On Tue, May 02, 2017 at 02:28:05PM -0600, Sameer Goel wrote:
> >>>>> Port architecture specific xlate and unxlate functions for /dev/mem
> >>>>> read/write. This sets up the mapping for a valid physical address if a
> >>>>> kernel direct mapping is not already present.
> >>>>>
> >>>>> This is a generic issue as a user space app should not be allowed to crash
> >>>>> the kernel.
> >>>>
> >>>>> This issue was observed when systemd tried to access performance
> >>>>> pointer record from the FPDT table.
> >>>>
> >>>> Why is it doing that? Is there not a way to get this via /sys?
> >>>
> >>> There is no ACPI FPDT implementation in the kernel, so the userspace
> >>> systemd code is getting the FPDT table contents from /sys
> >>> and parsing the entries. The performance pointer record is a
> >>> reserved address populated by UEFI and the userspace code tries to
> >>> access it using /dev/mem. The physical address is valid, so cannot
> >>> push back on the user space code.
> >>
> >> OK, so then we need to add support for parsing this table in the
> >> kernel and exposing the referred-to regions in a controllable fashion.
> >> Maybe something that belongs under /sys/firmware/efi (adding Matt), or
> >> maybe something that deserves its own driver.
> >>
> >> The only two use-cases for /dev/mem on arm64 are:
> >> - Implementing interfaces in the kernel takes up-front effort.
> >> - Being able to accidentally panic the kernel from userland.
> >>
> > We will see this issue with any access using /dev/mem to a MEMBLOCK_NOMAP marked
> > memblock. The kernel crash issue has to be fixed irrespective of ACPI FPDT support.
> >
> 
> I reported the same issue a couple of weeks ago [0]. So while we all
> agree that such accesses shouldn't oops the kernel, I think we may
> disagree on whether such accesses should be allowed in the first
> place, especially when using read/write on /dev/mem (as opposed to
> mmap())

Did you plan to respin those patches to address Alex's comments? I agree
that it would be good to close the oops, regardless of the rest of the
discussion here.

Will