On Sun, Dec 4, 2022 at 12:45 PM Dmitry Osipenko dmitry.osipenko@collabora.com wrote:

On 11/30/22 21:57, Rob Clark wrote:

...

From: Rob Clark robdclark@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@syzkaller.appspotmail.com Fixes: 2194a63a818d ("drm: Add library for shmem backed GEM objects") Cc: stable@vger.kernel.org Signed-off-by: Rob Clark robdclark@chromium.org Reviewed-by: Daniel Vetter daniel.vetter@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