[PATCH v2 1/2] drm/shmem-helper: Remove errant put in error path

[Rob Clark]

On Sun, Dec 4, 2022 at 12:45 PM Dmitry Osipenko
<dmitry.osipenko at collabora.com> wrote:
>
> On 11/30/22 21:57, Rob Clark wrote:
> > From: Rob Clark <robdclark at chromium.org>
> >
> > drm_gem_shmem_mmap() doesn't own this reference, resulting in the GEM
> > object getting prematurely freed leading to a later use-after-free.
> >
> > Link: https://syzkaller.appspot.com/bug?extid=c8ae65286134dd1b800d
> > Reported-by: syzbot+c8ae65286134dd1b800d at syzkaller.appspotmail.com
> > Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects")
> > Cc: stable at vger.kernel.org
> > Signed-off-by: Rob Clark <robdclark at chromium.org>
> > Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> > ---
> >  drivers/gpu/drm/drm_gem_shmem_helper.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > index 35138f8a375c..3b7b71391a4c 100644
> > --- a/drivers/gpu/drm/drm_gem_shmem_helper.c
> > +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
> > @@ -622,10 +622,8 @@ int drm_gem_shmem_mmap(struct drm_gem_shmem_object *shmem, struct vm_area_struct
> >       }
> >
> >       ret = drm_gem_shmem_get_pages(shmem);
> > -     if (ret) {
> > -             drm_gem_vm_close(vma);
> > +     if (ret)
> >               return ret;
> > -     }
> >
> >       vma->vm_flags |= VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP;
> >       vma->vm_page_prot = vm_get_page_prot(vma->vm_flags);
>
> AFAICS, the dmabuf mmaping code path needs a similar fix, isn't it?
>
> -               /* 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 = dma_buf_mmap(obj->dma_buf, vma, 0);
> +
> +               /* Drop the reference drm_gem_mmap_obj() acquired.*/
> +               if (!ret)
> +                       drm_gem_object_put(obj);
> +
> +               return ret;
>

Yes, it seems that way.. I wish the shmem helpers worked in a less
"special" way with regards to refcnting :-(

BR,
-R