Re: [PATCH] drm/shme-helpers: Fix dma_buf_mmap forwarding bug

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Oct 28, 2020 at 09:44:15AM +0100, Boris Brezillon wrote:
> On Tue, 27 Oct 2020 22:49:22 +0100
> Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> 
> > When we forward an mmap to the dma_buf exporter, they get to own
> > everything. Unfortunately drm_gem_mmap_obj() overwrote
> > vma->vm_private_data after the driver callback, wreaking the
> > exporter complete. This was noticed because vb2_common_vm_close blew
> > up on mali gpu with panfrost after commit 26d3ac3cb04d
> > ("drm/shmem-helpers: Redirect mmap for imported dma-buf").
> > 
> > Unfortunately drm_gem_mmap_obj also acquires a surplus reference that
> > we need to drop in shmem helpers, which is a bit of a mislayer
> > situation. Maybe the entire dma_buf_mmap forwarding should be pulled
> > into core gem code.
> > 
> > Note that the only two other drivers which forward mmap in their own
> > code (etnaviv and exynos) get this somewhat right by overwriting the
> > gem mmap code. But they seem to still have the leak. This might be a
> > good excuse to move these drivers over to shmem helpers completely.
> > 
> > Note to stable team: There's a trivial context conflict with
> > d693def4fd1c ("drm: Remove obsolete GEM and PRIME callbacks from
> > struct drm_driver"). I'm assuming it's clear where to put the first
> > hunk, otherwise I can send a 5.9 version of this.
> > 
> > Cc: Christian König <christian.koenig@amd.com>
> > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > Cc: Lucas Stach <l.stach@pengutronix.de>
> > Cc: Russell King <linux+etnaviv@armlinux.org.uk>
> > Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
> > Cc: Inki Dae <inki.dae@samsung.com>
> > Cc: Joonyoung Shim <jy0922.shim@samsung.com>
> > Cc: Seung-Woo Kim <sw0312.kim@samsung.com>
> > Cc: Kyungmin Park <kyungmin.park@samsung.com>
> > Fixes: 26d3ac3cb04d ("drm/shmem-helpers: Redirect mmap for imported dma-buf")
> > Cc: Boris Brezillon <boris.brezillon@collabora.com>
> 
> Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>

Patch pushed to drm-misc-fixes, thanks for taking a look.
-Daniel

> 
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: Gerd Hoffmann <kraxel@redhat.com>
> > Cc: Rob Herring <robh@kernel.org>
> > Cc: dri-devel@lists.freedesktop.org
> > Cc: linux-media@vger.kernel.org
> > Cc: linaro-mm-sig@lists.linaro.org
> > Cc: <stable@vger.kernel.org> # v5.9+
> > Reported-and-tested-by: piotr.oniszczuk@gmail.com
> > Cc: piotr.oniszczuk@gmail.com
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > ---
> >  drivers/gpu/drm/drm_gem.c              | 4 ++--
> >  drivers/gpu/drm/drm_gem_shmem_helper.c | 7 ++++++-
> >  2 files changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index 1da67d34e55d..d586068f5509 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -1076,6 +1076,8 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
> >  	 */
> >  	drm_gem_object_get(obj);
> >  
> > +	vma->vm_private_data = obj;
> > +
> >  	if (obj->funcs->mmap) {
> >  		ret = obj->funcs->mmap(obj, vma);
> >  		if (ret) {
> > @@ -1096,8 +1098,6 @@ int drm_gem_mmap_obj(struct drm_gem_object *obj, unsigned long obj_size,
> >  		vma->vm_page_prot = pgprot_decrypted(vma->vm_page_prot);
> >  	}
> >  
> > -	vma->vm_private_data = obj;
> > -
> >  	return 0;
> >  }
> >  EXPORT_SYMBOL(drm_gem_mmap_obj);
> > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > index fb11df7aced5..8233bda4692f 100644
> > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > @@ -598,8 +598,13 @@ int drm_gem_shmem_mmap(struct drm_gem_object *obj, struct vm_area_struct *vma)
> >  	/* Remove the fake offset */
> >  	vma->vm_pgoff -= drm_vma_node_start(&obj->vma_node);
> >  
> > -	if (obj->import_attach)
> > +	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);
> > +	}
> >  
> >  	shmem = to_drm_gem_shmem_obj(obj);
> >  
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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