Received-SPF: pass (google.com: domain of linux-wireless-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org 3BC48C433C6
From:   Kalle Valo <kvalo@codeaurora.org>
To:     Christian Lamparter <chunkeey@gmail.com>
Cc:     Jia-Ju Bai <baijiaju@tsinghua.edu.cn>, kuba@kernel.org,
        linux-wireless@vger.kernel.org, Vinod Koul <vkoul@kernel.org>
Subject: Re: [PATCH] p54: avoid accessing the data mapped to streaming DMA
References: <20200802132949.26788-1-baijiaju@tsinghua.edu.cn>
        <0b432282-8d52-3004-6648-d97c03cdae28@gmail.com>
        <873649cs4c.fsf@codeaurora.org>
        <a7878a0d-fe97-0c88-3c99-a08dab7be6bc@gmail.com>
Date:   Mon, 31 Aug 2020 17:00:10 +0300
In-Reply-To: <a7878a0d-fe97-0c88-3c99-a08dab7be6bc@gmail.com> (Christian
        Lamparter's message of "Fri, 28 Aug 2020 23:19:09 +0200")
Message-ID: <8736427w5x.fsf@codeaurora.org>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Sender: linux-wireless-owner@vger.kernel.org
Precedence: bulk

Christian Lamparter <chunkeey@gmail.com> writes:

> On 2020-08-26 18:02, Kalle Valo wrote:
>> Christian Lamparter <chunkeey@gmail.com> writes:
>>
>>> On 2020-08-02 15:29, Jia-Ju Bai wrote:
>>>> In p54p_tx(), skb->data is mapped to streaming DMA on line 337:
>>>>     mapping = pci_map_single(..., skb->data, ...);
>>>>
>>>> Then skb->data is accessed on line 349:
>>>>     desc->device_addr = ((struct p54_hdr *)skb->data)->req_id;
>>>>
>>>> This access may cause data inconsistency between CPU cache and hardware.
>>>>
>>>> To fix this problem, ((struct p54_hdr *)skb->data)->req_id is stored in
>>>> a local variable before DMA mapping, and then the driver accesses this
>>>> local variable instead of skb->data.
>>>
>>> Interesting. Please bear with me here. From my understanding, the
>>> streaming direction is set to PCI_DMA_TODEVICE. So is it really
>>> possible for the hardware to interfere with the data without the IOMMU
>>> catching this?
>>
>> Also is there any documentation about this scenario? I would like to
>> understand this better.
>
> I usually rely on the information present in Documentation:
> <https://www.kernel.org/doc/Documentation/DMA-API-HOWTO.txt>
>
> The relevant extract for p54's DMA_TO_DEVICE decision likely comes from:
>
> "For Networking drivers, it's a rather simple affair.  For transmit
> packets, map/unmap them with the DMA_TO_DEVICE direction
> specifier.  For receive packets, just the opposite, map/unmap them
> with the DMA_FROM_DEVICE direction specifier."
>
> "Only streaming mappings specify a direction, consistent mappings
> implicitly have a direction attribute setting of DMA_BIDIRECTIONAL."

This is not very clearly written, I guess it's assumed everyone know
this stuff :)

> But looking around on the Internet, I came across this in "Chapter 15.
> Memory Mapping and DMA" of the Linux Device Drivers, 3rd Edition:
>
> <https://www.oreilly.com/library/view/linux-device-drivers/0596005903/ch15.html>
>
> |Setting up streaming DMA mappings
> |[...]
> |
> |Some important rules apply to streaming DMA mappings:
> | * [...]
> |
> | * Once a buffer has been mapped, it belongs to the device, not the
> |   processor. Until the buffer has been unmapped, the driver should
> not |   touch its contents in any way. Only after dma_unmap_single has
> been |   called is it safe for the driver to access the contents of
> the
> |   buffer (with one exception that we see shortly). Among other things,
> |   this rule implies that a buffer being written to a device cannot be
> |   mapped until it contains all the data to write."
> |
> | [...] (More informative text, but only)
>
> From the sentence "Once a buffer has been mapped, it belongs to the
> device, not the processor". I think that Jia-Ju Bai's patch is doing
> exactly this "by the book".

Yeah, this is much better and understandable. Thanks for checking.

> Therefore, it should be applied and backported:
>
> Cc: <stable@vger.kernel.org>

Ok, I'll add that.

-- 
https://wireless.wiki.kernel.org/en/developers/documentation/submittingpatches