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

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 7/6/22 00:48, Rob Clark wrote:
> On Tue, Jul 5, 2022 at 4:51 AM Christian König <christian.koenig@amd.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@linux.intel.com>
>>> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.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