Date:   Thu, 26 Jul 2018 11:44:25 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Will Deacon <will.deacon@....com>,
        "Joerg Roedel" <joro@...tes.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        LinuxArm <linuxarm@...wei.com>
Subject: Re: [PATCH v3 0/6] add non-strict mode support for arm-smmu-v3



On 2018/7/25 5:51, Robin Murphy wrote:
> On 2018-07-12 7:18 AM, Zhen Lei wrote:
>> v2 -> v3:
>> Add a bootup option "iommu_strict_mode" to make the manager can choose which
>> mode to be used. The first 5 patches have not changed.
>> +    iommu_strict_mode=    [arm-smmu-v3]
>> +        0 - strict mode (default)
>> +        1 - non-strict mode
>>
>> v1 -> v2:
>> Use the lowest bit of the io_pgtable_ops.unmap's iova parameter to pass the strict mode:
>> 0, IOMMU_STRICT;
>> 1, IOMMU_NON_STRICT;
>> Treat 0 as IOMMU_STRICT, so that the unmap operation can compatible with
>> other IOMMUs which still use strict mode. In other words, this patch series
>> will not impact other IOMMU drivers. I tried add a new quirk IO_PGTABLE_QUIRK_NON_STRICT
>> in io_pgtable_cfg.quirks, but it can not pass the strict mode of the domain from SMMUv3
>> driver to io-pgtable module.
> 
> What exactly is the issue there? We don't have any problem with other quirks like NO_DMA, and as I said before, by the time we're allocating the io-pgtable in arm_smmu_domain_finalise() we already know everything there is to know about the domain.

Because userspace can map/unamp and start devices to access memory through VFIO.
So that, the attacker can:
1. alloc memory
2. map
3. unmap
4. free memory
5. repeatedly accesssing the just freed memory base on the just unmapped iova,
   this attack may success if the freed memory is reused by others and the mapping still staying in TLB

But if only root user can use VFIO, this is an unnecessary worry. Then both normal and VFIO will use the
same strict mode, so that the new quirk IO_PGTABLE_QUIRK_NON_STRICT can easily be applied.

> 
>> Add a new member domain_non_strict in struct iommu_dma_cookie, this member will only be
>> initialized when the related domain and IOMMU driver support non-strict mode.
>>
>> v1:
>> In common, a IOMMU unmap operation follow the below steps:
>> 1. remove the mapping in page table of the specified iova range
>> 2. execute tlbi command to invalid the mapping which is cached in TLB
>> 3. wait for the above tlbi operation to be finished
>> 4. free the IOVA resource
>> 5. free the physical memory resource
>>
>> This maybe a problem when unmap is very frequently, the combination of tlbi
>> and wait operation will consume a lot of time. A feasible method is put off
>> tlbi and iova-free operation, when accumulating to a certain number or
>> reaching a specified time, execute only one tlbi_all command to clean up
>> TLB, then free the backup IOVAs. Mark as non-strict mode.
>>
>> But it must be noted that, although the mapping has already been removed in
>> the page table, it maybe still exist in TLB. And the freed physical memory
>> may also be reused for others. So a attacker can persistent access to memory
>> based on the just freed IOVA, to obtain sensible data or corrupt memory. So
>> the VFIO should always choose the strict mode.
>>
>> Some may consider put off physical memory free also, that will still follow
>> strict mode. But for the map_sg cases, the memory allocation is not controlled
>> by IOMMU APIs, so it is not enforceable.
>>
>> Fortunately, Intel and AMD have already applied the non-strict mode, and put
>> queue_iova() operation into the common file dma-iommu.c., and my work is based
>> on it. The difference is that arm-smmu-v3 driver will call IOMMU common APIs to
>> unmap, but Intel and AMD IOMMU drivers are not.
>>
>> Below is the performance data of strict vs non-strict for NVMe device:
>> Randomly Read  IOPS: 146K(strict) vs 573K(non-strict)
>> Randomly Write IOPS: 143K(strict) vs 513K(non-strict)
> 
> How does that compare to passthrough performance? One thing I'm not entirely clear about is what the realistic use-case for this is - even if invalidation were infinitely fast, enabling translation still typically has a fair impact on overall system performance in terms of latency, power, memory bandwidth, etc., so I can't help wonder what devices exist today for which performance is critical and robustness* is unimportant, yet have crippled addressing capabilities such that they can't just use passthrough.
I have no passthrough performance data yet, I will ask my team to do it. But we have tested the Global bypass:
Randomly Read IOPS: 744K, and Randomly Write IOPS: is the same to non-strict.

I'm also not clear. But I think in most cases, the system does not need to run at full capacity, but the system
should have that ability. For example, a system's daily load may only 30-50%, but the load may increase to 80%+
on festival day.

Passthrough is not enough to support VFIO, and virtualization need the later.

> 
> Robin.
> 
> 
> * I don't want to say "security" here, since I'm actually a lot less concerned about the theoretical malicious endpoint/wild write scenarios than the the much more straightforward malfunctioning device and/or buggy driver causing use-after-free style memory corruption. Also, I'm sick of the word "security"...

OK，We really have no need to consider buggy devices.

> 
>>
>> Zhen Lei (6):
>>    iommu/arm-smmu-v3: fix the implementation of flush_iotlb_all hook
>>    iommu/dma: add support for non-strict mode
>>    iommu/amd: use default branch to deal with all non-supported
>>      capabilities
>>    iommu/io-pgtable-arm: add support for non-strict mode
>>    iommu/arm-smmu-v3: add support for non-strict mode
>>    iommu/arm-smmu-v3: add bootup option "iommu_strict_mode"
>>
>>   Documentation/admin-guide/kernel-parameters.txt | 12 +++++++
>>   drivers/iommu/amd_iommu.c                       |  4 +--
>>   drivers/iommu/arm-smmu-v3.c                     | 42 +++++++++++++++++++++++--
>>   drivers/iommu/dma-iommu.c                       | 25 +++++++++++++++
>>   drivers/iommu/io-pgtable-arm.c                  | 23 ++++++++------
>>   include/linux/iommu.h                           |  7 +++++
>>   6 files changed, 98 insertions(+), 15 deletions(-)
>>
> 
> .
> 

-- 
Thanks!
BestRegards

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