[RFC PATCH v2 0/8] ACPI/IORT: Support for IORT RMR node

[Shameerali Kolothum Thodi]

> -----Original Message-----
> From: Steven Price [mailto:steven.price at arm.com]
> Sent: 14 December 2020 13:43
> To: Robin Murphy <robin.murphy at arm.com>; Shameerali Kolothum Thodi
> <shameerali.kolothum.thodi at huawei.com>;
> linux-arm-kernel at lists.infradead.org; linux-acpi at vger.kernel.org;
> iommu at lists.linux-foundation.org; devel at acpica.org
> Cc: Linuxarm <linuxarm at huawei.com>; lorenzo.pieralisi at arm.com;
> joro at 8bytes.org; wanghuiqiang <wanghuiqiang at huawei.com>; Guohanjun
> (Hanjun Guo) <guohanjun at huawei.com>; Jonathan Cameron
> <jonathan.cameron at huawei.com>; Sami.Mujawar at 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 at arm.com]
> >>> Sent: 10 December 2020 10:26
> >>> To: Shameerali Kolothum Thodi
> <shameerali.kolothum.thodi at huawei.com>;
> >>> linux-arm-kernel at lists.infradead.org; linux-acpi at vger.kernel.org;
> >>> iommu at lists.linux-foundation.org; devel at acpica.org
> >>> Cc: Linuxarm <linuxarm at huawei.com>; lorenzo.pieralisi at arm.com;
> >>> joro at 8bytes.org; robin.murphy at arm.com; wanghuiqiang
> >>> <wanghuiqiang at huawei.com>; Guohanjun (Hanjun Guo)
> >>> <guohanjun at huawei.com>; Jonathan Cameron
> >>> <jonathan.cameron at huawei.com>; Sami.Mujawar at 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.

Thanks,
Shameer