linux-kernel - Re: [PATCH] tpm: of: avoid __va() translation for event log address

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Date:   Mon, 28 Sep 2020 07:56:07 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-integrity <linux-integrity@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org, Ard Biesheuvel <ardb@...nel.org>,
        Peter Huewe <peterhuewe@....de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: of: avoid __va() translation for event log address



Le 28/09/2020 à 01:44, Jarkko Sakkinen a écrit :
> On Fri, Sep 25, 2020 at 09:00:18AM -0300, Jason Gunthorpe wrote:
>> On Fri, Sep 25, 2020 at 01:29:20PM +0300, Jarkko Sakkinen wrote:
>>> On Fri, Sep 25, 2020 at 09:00:56AM +0200, Ard Biesheuvel wrote:
>>>> On Fri, 25 Sep 2020 at 07:56, Jarkko Sakkinen
>>>> <jarkko.sakkinen@...ux.intel.com> wrote:
>>>>>
>>>>> On Tue, Sep 22, 2020 at 11:41:28AM +0200, Ard Biesheuvel wrote:
>>>>>> The TPM event log is provided to the OS by the firmware, by loading
>>>>>> it into an area in memory and passing the physical address via a node
>>>>>> in the device tree.
>>>>>>
>>>>>> Currently, we use __va() to access the memory via the kernel's linear
>>>>>> map: however, it is not guaranteed that the linear map covers this
>>>>>> particular address, as we may be running under HIGHMEM on a 32-bit
>>>>>> architecture, or running firmware that uses a memory type for the
>>>>>> event log that is omitted from the linear map (such as EfiReserved).
>>>>>
>>>>> Makes perfect sense to the level that I wonder if this should have a
>>>>> fixes tag and/or needs to be backported to the stable kernels?
>>>>>
>>>>
>>>> AIUI, the code was written specifically for ppc64, which is a
>>>> non-highmem, non-EFI architecture. However, when we start reusing this
>>>> driver for ARM, this issue could pop up.
>>>>
>>>> The code itself has been refactored a couple of times, so I think it
>>>> will require different versions of the patch for different generations
>>>> of stable kernels.
>>>>
>>>> So perhaps just add Cc: <stable@...r.kernel.org>, and wait and see how
>>>> far back it applies cleanly?
>>>
>>> Yeah, I think I'll cc it with some note before the diffstat.
>>>
>>> I'm thinking to cap it to only 5.x kernels (at least first) unless it is
>>> dead easy to backport below that.
>>
>> I have this vauge recollection of pointing at this before and being
>> told that it had to be __va for some PPC reason?
>>
>> Do check with the PPC people first, I see none on the CC list.
>>
>> Jason
> 
> Thanks, added arch/powerpc maintainers.
> 

As far as I can see, memremap() won't work on PPC32 at least:

IIUC, memremap() calls arch_memremap_wb()
arch_memremap_wb() calls ioremap_cache()
In case of failure, then ioremap_wt() and ioremap_wc() are tried.

All ioremap calls end up in __ioremap_caller() which will return NULL in case you try to ioremap RAM.

So the statement "So instead, use memremap(), which will reuse the linear mapping if
it is valid, or create another mapping otherwise." seems to be wrong, at least for PPC32.

Even for PPC64 which doesn't seem to have the RAM check, I can't see that it will "reuse the linear 
mapping".

Christophe