linux-kernel - Re: [PATCH v4 08/35] iommu/mediatek: Use kmalloc for protect buffer

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Date:   Wed, 16 Feb 2022 14:59:05 +0900
From:   Tomasz Figa <tfiga@...omium.org>
To:     Yong Wu <Yong.Wu@...iatek.com>
Cc:     AngeloGioacchino Del Regno 
        <angelogioacchino.delregno@...labora.com>,
        Robin Murphy <robin.murphy@....com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        linux-mediatek@...ts.infradead.org, srv_heupstream@...iatek.com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org,
        Hsin-Yi Wang <hsinyi@...omium.org>, youlin.pei@...iatek.com,
        anan.sun@...iatek.com, xueqi.zhang@...iatek.com,
        yen-chang.chen@...iatek.com, mingyuan.ma@...iatek.com,
        yf.wang@...iatek.com, libo.kang@...iatek.com,
        chengci.xu@...iatek.com, Joerg Roedel <joro@...tes.org>,
        Rob Herring <robh+dt@...nel.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH v4 08/35] iommu/mediatek: Use kmalloc for protect buffer

On Wed, Feb 16, 2022 at 2:55 PM Yong Wu <yong.wu@...iatek.com> wrote:
>
> On Thu, 2022-01-27 at 12:08 +0100, AngeloGioacchino Del Regno wrote:
> > Il 25/01/22 09:56, Yong Wu ha scritto:
> > > No need zero for the protect buffer that is only accessed by the
> > > IOMMU HW
> > > translation fault happened.
> > >
> > > Signed-off-by: Yong Wu <yong.wu@...iatek.com>
> >
> > I would rather keep this a devm_kzalloc instead... the cost is very
> > minimal and
> > this will be handy when new hardware will be introduced, as it may
> > require a bigger
> > buffer: in that case, "older" platforms will use only part of it and
> > we may get
> > garbage data at the end.
>
> Currently this is to avoid zero 512 bytes for all the platforms.
>
> Sorry, I don't understand why it is unnecessary when the new hardware
> requires a bigger buffer. If the buffer becomes bigger, then clearing
> it to 0 need more cost. then this patch is more helpful?
>
> The content in this buffer is garbage, we won't care about or analyse
> it.

I think we should zero it for security reasons regardless of any other
aspects. With this patch it's leaking kernel data to the hardware.

At the same time, we're talking here about something executed just 1
time when the driver probes. I don't think the cost would really
matter.

Best regards,
Tomasz