Re: [PATCH v6 02/22] drm/gem: Move mapping of imported dma-bufs to drm_gem_mmap_obj()

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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@xxxxxxxxxxxxxxx
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
---
  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;



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