RE: [RFC PATCH v2 0/8] ACPI/IORT: Support for IORT RMR node | linux-arm-kernel

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-----Original Message-----
From: Jon Nettleton [mailto:jon@solid-run.com]
Sent: 17 December 2020 14:48
To: Shameerali Kolothum Thodi <redacted>
Cc: Steven Price <steven.price@arm.com>; Robin Murphy
[off-list ref]; linux-arm-kernel@lists.infradead.org;
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Jonathan Cameron [off-list ref];
Sami.Mujawar@arm.com; wanghuiqiang [off-list ref]
Subject: Re: [RFC PATCH v2 0/8] ACPI/IORT: Support for IORT RMR node

On Mon, Dec 14, 2020 at 3:48 PM Shameerali Kolothum Thodi
[off-list ref] wrote:

-----Original Message-----
From: Steven Price [mailto:steven.price@arm.com]
Sent: 14 December 2020 13:43
To: Robin Murphy <robin.murphy@arm.com>; Shameerali Kolothum Thodi
[off-list ref];
linux-arm-kernel@lists.infradead.org; linux-acpi@vger.kernel.org;
iommu@lists.linux-foundation.org; devel@acpica.org
Cc: Linuxarm <redacted>; lorenzo.pieralisi@arm.com;
joro@8bytes.org; wanghuiqiang [off-list ref]; Guohanjun
(Hanjun Guo) [off-list ref]; Jonathan Cameron
[off-list ref]; Sami.Mujawar@arm.com
Subject: Re: [RFC PATCH v2 0/8] ACPI/IORT: Support for IORT RMR node

On 14/12/2020 12:33, Robin Murphy wrote:
On 2020-12-14 10:55, Shameerali Kolothum Thodi wrote:
Hi Steve,

-----Original Message-----
From: Steven Price [mailto:steven.price@arm.com]
Sent: 10 December 2020 10:26
To: Shameerali Kolothum Thodi
[off-list ref];
linux-arm-kernel@lists.infradead.org; linux-acpi@vger.kernel.org;
iommu@lists.linux-foundation.org; devel@acpica.org
Cc: Linuxarm <redacted>; lorenzo.pieralisi@arm.com;
joro@8bytes.org; robin.murphy@arm.com; wanghuiqiang
[off-list ref]; Guohanjun (Hanjun Guo)
[off-list ref]; Jonathan Cameron
[off-list ref]; Sami.Mujawar@arm.com
Subject: Re: [RFC PATCH v2 0/8] ACPI/IORT: Support for IORT RMR node

On 19/11/2020 12:11, Shameer Kolothum wrote:
RFC v1 --> v2:
   - Added a generic interface for IOMMU drivers to retrieve all the
     RMR info associated with a given IOMMU.
   - SMMUv3 driver gets the RMR list during probe() and installs
     bypass STEs for all the SIDs in the RMR list. This is to keep
     the ongoing traffic alive(if any) during SMMUv3 reset. This is
     based on the suggestions received for v1 to take care of the
     EFI framebuffer use case. Only sanity tested for now.
Hi Shameer,

Sorry for not looking at this before.

Do you have any plans to implement support in the SMMUv2 driver?
The
platform I've been testing the EFI framebuffer support on has the
display controller behind SMMUv2, so as it stands this series doesn't
work. I did hack something up for SMMUv2 so I was able to test the
first
4 patches.
Thanks for taking a look. Sure, I can look into adding the support for
SMMUv2.
Great, thanks!

   - During the probe/attach device, SMMUv3 driver reserves any
     RMR region associated with the device such that there is a
unity
     mapping for them in SMMU.
For the EFI framebuffer use case there is no device to attach so I
believe we are left with just the stream ID in bypass mode - which is
definitely an improvement (the display works!)
Cool. That’s good to know.

  but not actually a unity
mapping of the RMR range. I'm not sure whether it's worth fixing this
or
not, but I just wanted to point out there's still a need for a driver
for the device before the bypass mode is replaced with the unity
mapping.
I am not sure either. My idea was we will have bypass STE setup for
all devices
with RMR initially and when the corresponding driver takes over(if
that happens)
we will have the unity mapping setup properly for the RMR regions. And
for cases
like the above, it will remain in the bypass mode.

Do you see any problem(security?) if the dev streams remain in bypass
mode for
this dev? Or is it possible to have a stub driver for this dev, so
that we will have
the probe/attach invoked and everything will fall in place?
The downside is that if a driver never binds to that device, it remains
bypassed. If some other externally-controlled malicious device could
manage to spoof that device's requester ID, that could potentially be
exploited.

TBH, I haven't looked into creating a temp domain for these types of
the devices
and also not sure how we benefit from that compared to the STE bypass
mode.
That said, setting up temporary translation contexts that ensure any
access can *only* be to RMR regions until a driver takes over is an
awful lot more work. I'm inclined to be pragmatic here and say we should
get things working at all with the simple bypass STE/S2CR method, then
look at adding the higher-security effort on top.

Right now systems that need this are either broken (but effectively
secure) or using default bypass with SMMUv2. People who prefer to
maintain security over functionality in the interim can maintain that
status quo by simply continuing to not describe any RMRs.
I agree with Robin, let's get this working with the bypass mode (until
the device binds) like you've currently got. It's much better than what
we have otherwise. Then once that is merged we can look at the temporary
translation context or stub driver. The temporary translation context
would be 'neatest', but I'm only aware of the EFI framebuffer use case
and for that it might be possible to do something simpler - if indeed
anything is needed at all. I'm not sure how much we need to be worried
about malicious devices spoofing requester IDs.
Perfect. I will keep the STE bypass and respin the series once the update
to the IORT rev E is made public(hope that will happen soon). In the
meantime, appreciate any feedback on the rest of the patches in this series.
Shameer,
Hi Jon,

I am pretty sure rev E is already public.  You can find it here.

https://developer.arm.com/documentation/den0049/latest/

It is also marked non-confidential.
Yes, Rev E is already out there. But I am told that ARM folks are working on
some updates to the IORT spec, especially around the RMR topic. Hopefully
it will be out soon.

I also have initial patches for edk2 and the HoneyComb LX2160a
ACPI tables adding RMR nodes that partially work with your patches.
Thanks for the update and good to know that it is useful.

Shameer

This is with basic SMMUv2 support but since you have more experience
this this I am more than happy to work with you on your patchset.

-Jon

Thanks,
Shameer
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