linux-kernel - Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

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Message-ID: <2a646ce4-c2ec-3b11-77a0-cc720afd6fe1@collabora.com>
Date:   Wed, 6 Jul 2022 10:02:07 +0300
From:   Dmitry Osipenko <dmitry.osipenko@...labora.com>
To:     Rob Clark <robdclark@...il.com>,
        Christian König <christian.koenig@....com>
Cc:     David Airlie <airlied@...ux.ie>, Gerd Hoffmann <kraxel@...hat.com>,
        Gurchetan Singh <gurchetansingh@...omium.org>,
        Chia-I Wu <olvaffe@...il.com>, Daniel Vetter <daniel@...ll.ch>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Thomas Hellström <thomas_os@...pmail.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        linux-tegra@...r.kernel.org, Dmitry Osipenko <digetx@...il.com>,
        kernel@...labora.com
Subject: Re: [PATCH v8 2/2] drm/gem: Don't map imported GEMs

On 7/6/22 00:48, Rob Clark wrote:
> On Tue, Jul 5, 2022 at 4:51 AM Christian König <christian.koenig@....com> wrote:
>>
>> Am 01.07.22 um 11:02 schrieb Dmitry Osipenko:
>>> Drivers that use drm_gem_mmap() and drm_gem_mmap_obj() helpers don't
>>> handle imported dma-bufs properly, which results in mapping of something
>>> else than the imported dma-buf. On NVIDIA Tegra we get a hard lockup when
>>> userspace writes to the memory mapping of a dma-buf that was imported into
>>> Tegra's DRM GEM.
>>>
>>> Majority of DRM drivers prohibit mapping of the imported GEM objects.
>>> Mapping of imported GEMs require special care from userspace since it
>>> should sync dma-buf because mapping coherency of the exporter device may
>>> not match the DRM device. Let's prohibit the mapping for all DRM drivers
>>> for consistency.
>>>
>>> Suggested-by: Thomas Hellström <thomas.hellstrom@...ux.intel.com>
>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
>>
>> I'm pretty sure that this is the right approach, but it's certainly more
>> than possible that somebody abused this already.
> 
> I suspect that this is abused if you run deqp cts on android.. ie. all
> winsys buffers are dma-buf imports from gralloc.  And then when you
> hit readpix...
> 
> You might only hit this in scenarios with separate gpu and display (or
> dGPU+iGPU) because self-imports are handled differently in
> drm_gem_prime_import_dev().. and maybe not in cases where you end up
> with a blit from tiled/compressed to linear.. maybe that narrows the
> scope enough to just fix it in userspace?

Given that that only drivers which use DRM-SHMEM potentially could've
map imported dma-bufs (Panfrost, Lima) and they already don't allow to
do that, I think we're good.

-- 
Best regards,
Dmitry