linux-kernel - Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap

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Message-ID: <b8271f0c-d6a3-4194-1959-e112859756a3@shipmail.org>
Date:   Wed, 29 Jun 2022 08:40:17 +0200
From:   Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
To:     Dmitry Osipenko <dmitry.osipenko@...labora.com>,
        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>,
        Daniel Almeida <daniel.almeida@...labora.com>,
        Gert Wollny <gert.wollny@...labora.com>,
        Gustavo Padovan <gustavo.padovan@...labora.com>,
        Daniel Stone <daniel@...ishbar.org>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Rob Herring <robh@...nel.org>,
        Steven Price <steven.price@....com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Rob Clark <robdclark@...il.com>,
        Emil Velikov <emil.l.velikov@...il.com>,
        Robin Murphy <robin.murphy@....com>,
        Qiang Yu <yuq825@...il.com>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Christian König <christian.koenig@....com>,
        "Pan, Xinhui" <Xinhui.Pan@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        Tomasz Figa <tfiga@...omium.org>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Tvrtko Ursulin <tvrtko.ursulin@...ux.intel.com>
Cc:     intel-gfx@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        linaro-mm-sig@...ts.linaro.org, amd-gfx@...ts.freedesktop.org,
        linux-tegra@...r.kernel.org, Dmitry Osipenko <digetx@...il.com>,
        kernel@...labora.com, linux-media@...r.kernel.org
Subject: Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to
 drm_gem_mmap_obj()


On 5/27/22 01:50, Dmitry Osipenko wrote:
> 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. For example, 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.
>
> To fix this bug, move mapping of imported dma-bufs to drm_gem_mmap_obj().
> Now mmaping of imported dma-bufs works properly for all DRM drivers.
Same comment about Fixes: as in patch 1,
>
> Cc: stable@...r.kernel.org
> Signed-off-by: Dmitry Osipenko <dmitry.osipenko@...labora.com>
> ---
>   drivers/gpu/drm/drm_gem.c              | 3 +++
>   drivers/gpu/drm/drm_gem_shmem_helper.c | 9 ---------
>   drivers/gpu/drm/tegra/gem.c            | 4 ++++
>   3 files changed, 7 insertions(+), 9 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index 86d670c71286..7c0b025508e4 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -1038,6 +1038,9 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
>   	if (obj_size < vma->vm_end - vma->vm_start)
>   		return -EINVAL;
>   
> +	if (obj->import_attach)
> +		return dma_buf_mmap(obj->dma_buf, vma, 0);

If we start enabling mmaping of imported dma-bufs on a majority of 
drivers in this way, how do we ensure that user-space is not blindly 
using the object mmap without calling the needed DMA_BUF_IOCTL_SYNC 
which is needed before and after cpu access of mmap'ed dma-bufs?

I was under the impression (admittedly without looking) that the few 
drivers that actually called into dma_buf_mmap() had some private 
user-mode driver code in place that ensured this happened.

/Thomas


> +
>   	/* Take a ref for this mapping of the object, so that the fault
>   	 * handler can dereference the mmap offset's pointer to the object.
>   	 * This reference is cleaned up by the corresponding vm_close
> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> index 8ad0e02991ca..6190f5018986 100644
> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> @@ -609,17 +609,8 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_vm_ops);
>    */
>   int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct *vma)
>   {
> -	struct drm_gem_object *obj = &shmem->base;
>   	int ret;
>   
> -	if (obj->import_attach) {
> -		/* Drop the reference drm_gem_mmap_obj() acquired.*/
> -		drm_gem_object_put(obj);
> -		vma->vm_private_data = NULL;
> -
> -		return dma_buf_mmap(obj->dma_buf, vma, 0);
> -	}
> -
>   	ret = drm_gem_shmem_get_pages(shmem);
>   	if (ret) {
>   		drm_gem_vm_close(vma);
> diff --git a/drivers/gpu/drm/tegra/gem.c b/drivers/gpu/drm/tegra/gem.c
> index 7c7dd84e6db8..f92aa20d63bb 100644
> --- a/drivers/gpu/drm/tegra/gem.c
> +++ b/drivers/gpu/drm/tegra/gem.c
> @@ -564,6 +564,10 @@ int __tegra_gem_mmap(struct drm_gem_object *gem, struct vm_area_struct *vma)
>   {
>   	struct tegra_bo *bo = to_tegra_bo(gem);
>   
> +	/* imported dmu-buf is mapped by drm_gem_mmap_obj()  */
> +	if (gem->import_attach)
> +		return 0;
> +
>   	if (!bo->pages) {
>   		unsigned long vm_pgoff = vma->vm_pgoff;
>   		int err;