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Subject: Re: [RFC PATCH v3 0/6] Restricted DMA
To:     Tomasz Figa <tfiga@chromium.org>
Cc:     Claire Chang <tientzu@chromium.org>,
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From:   Florian Fainelli <f.fainelli@gmail.com>
Message-ID: <7fe99ad2-79a7-9c8b-65ce-ce8353e9d9bf@gmail.com>
Date:   Tue, 12 Jan 2021 20:41:24 -0800
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On 1/12/2021 8:25 PM, Tomasz Figa wrote:
> On Wed, Jan 13, 2021 at 12:56 PM Florian Fainelli <f.fainelli@gmail.com> wrote:
>>
>>
>>
>> On 1/12/2021 6:29 PM, Tomasz Figa wrote:
>>> Hi Florian,
>>>
>>> On Wed, Jan 13, 2021 at 3:01 AM Florian Fainelli <f.fainelli@gmail.com> wrote:
>>>>
>>>> On 1/11/21 11:48 PM, Claire Chang wrote:
>>>>> On Fri, Jan 8, 2021 at 1:59 AM Florian Fainelli <f.fainelli@gmail.com> wrote:
>>>>>>
>>>>>> On 1/7/21 9:42 AM, Claire Chang wrote:
>>>>>>
>>>>>>>> Can you explain how ATF gets involved and to what extent it does help,
>>>>>>>> besides enforcing a secure region from the ARM CPU's perpsective? Does
>>>>>>>> the PCIe root complex not have an IOMMU but can somehow be denied access
>>>>>>>> to a region that is marked NS=0 in the ARM CPU's MMU? If so, that is
>>>>>>>> still some sort of basic protection that the HW enforces, right?
>>>>>>>
>>>>>>> We need the ATF support for memory MPU (memory protection unit).
>>>>>>> Restricted DMA (with reserved-memory in dts) makes sure the predefined memory
>>>>>>> region is for PCIe DMA only, but we still need MPU to locks down PCIe access to
>>>>>>> that specific regions.
>>>>>>
>>>>>> OK so you do have a protection unit of some sort to enforce which region
>>>>>> in DRAM the PCIE bridge is allowed to access, that makes sense,
>>>>>> otherwise the restricted DMA region would only be a hint but nothing you
>>>>>> can really enforce. This is almost entirely analogous to our systems then.
>>>>>
>>>>> Here is the example of setting the MPU:
>>>>> https://github.com/ARM-software/arm-trusted-firmware/blob/master/plat/mediatek/mt8183/drivers/emi_mpu/emi_mpu.c#L132
>>>>>
>>>>>>
>>>>>> There may be some value in standardizing on an ARM SMCCC call then since
>>>>>> you already support two different SoC vendors.
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> On Broadcom STB SoCs we have had something similar for a while however
>>>>>>>> and while we don't have an IOMMU for the PCIe bridge, we do have a a
>>>>>>>> basic protection mechanism whereby we can configure a region in DRAM to
>>>>>>>> be PCIe read/write and CPU read/write which then gets used as the PCIe
>>>>>>>> inbound region for the PCIe EP. By default the PCIe bridge is not
>>>>>>>> allowed access to DRAM so we must call into a security agent to allow
>>>>>>>> the PCIe bridge to access the designated DRAM region.
>>>>>>>>
>>>>>>>> We have done this using a private CMA area region assigned via Device
>>>>>>>> Tree, assigned with a and requiring the PCIe EP driver to use
>>>>>>>> dma_alloc_from_contiguous() in order to allocate from this device
>>>>>>>> private CMA area. The only drawback with that approach is that it
>>>>>>>> requires knowing how much memory you need up front for buffers and DMA
>>>>>>>> descriptors that the PCIe EP will need to process. The problem is that
>>>>>>>> it requires driver modifications and that does not scale over the number
>>>>>>>> of PCIe EP drivers, some we absolutely do not control, but there is no
>>>>>>>> need to bounce buffer. Your approach scales better across PCIe EP
>>>>>>>> drivers however it does require bounce buffering which could be a
>>>>>>>> performance hit.
>>>>>>>
>>>>>>> Only the streaming DMA (map/unmap) needs bounce buffering.
>>>>>>
>>>>>> True, and typically only on transmit since you don't really control
>>>>>> where the sk_buff are allocated from, right? On RX since you need to
>>>>>> hand buffer addresses to the WLAN chip prior to DMA, you can allocate
>>>>>> them from a pool that already falls within the restricted DMA region, right?
>>>>>>
>>>>>
>>>>> Right, but applying bounce buffering to RX will make it more secure.
>>>>> The device won't be able to modify the content after unmap. Just like what
>>>>> iommu_unmap does.
>>>>
>>>> Sure, however the goals of using bounce buffering equally applies to RX
>>>> and TX in that this is the only layer sitting between a stack (block,
>>>> networking, USB, etc.) and the underlying device driver that scales well
>>>> in order to massage a dma_addr_t to be within a particular physical range.
>>>>
>>>> There is however room for improvement if the drivers are willing to
>>>> change their buffer allocation strategy. When you receive Wi-Fi frames
>>>> you need to allocate buffers for the Wi-Fi device to DMA into, and that
>>>> happens ahead of the DMA transfers by the Wi-Fi device. At buffer
>>>> allocation time you could very well allocate these frames from the
>>>> restricted DMA region without having to bounce buffer them since the
>>>> host CPU is in control over where and when to DMA into.
>>>>
>>>
>>> That is, however, still a trade-off between saving that one copy and
>>> protection from the DMA tampering with the packet contents when the
>>> kernel is reading them. Notice how the copy effectively makes a
>>> snapshot of the contents, guaranteeing that the kernel has a
>>> consistent view of the packet, which is not true if the DMA could
>>> modify the buffer contents in the middle of CPU accesses.
>>
>> I would say that the window just became so much narrower for the PCIe
>> end-point to overwrite contents with the copy because it would have to
>> happen within the dma_unmap_{page,single} time and before the copy is
>> finished to the bounce buffer.
> 
> Not only. Imagine this:
> 
> a) Without bouncing:
> 
> - RX interrupt
> - Pass the packet to the network stack
> - Network stack validates the packet
> - DMA overwrites the packet
> - Network stack goes boom, because the packet changed after validation
> 
> b) With bouncing:
> 
> - RX interrupt
> - Copy the packet to a DMA-inaccessible buffer
> - Network stack validates the packet
> - Network stack is happy, because the packet is guaranteed to stay the
> same after validation

Yes that's a much safer set of operations, thanks for walking through a
practical example.
-- 
Florian