drm/vmwgfx: Fix up user_dmabuf refcounting

If user space calls unreference on a user_dmabuf it will typically
kill the struct ttm_base_object member which is responsible for the
user-space visibility. However the dmabuf part may still be alive and
refcounted. In some situations, like for shared guest-backed surface
referencing/opening, the driver may try to reference the
struct ttm_base_object member again, causing an immediate kernel warning
and a later kernel NULL pointer dereference.

Fix this by always maintaining a reference on the struct
ttm_base_object member, in situations where it might subsequently be
referenced.

Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Sinclair Yeh <syeh@vmware.com>

1 files changed, 21 insertions, 8 deletions

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c b/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c
index c1912f852b42..e57667ca7557 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_resource.c
@@ -354,7 +354,7 @@ int vmw_user_lookup_handle(struct vmw_private *dev_priv,
 	}
 
 	*out_surf = NULL;
-	ret = vmw_user_dmabuf_lookup(tfile, handle, out_buf);
+	ret = vmw_user_dmabuf_lookup(tfile, handle, out_buf, NULL);
 	return ret;
 }
 
@@ -481,7 +481,8 @@ int vmw_user_dmabuf_alloc(struct vmw_private *dev_priv,
 			  uint32_t size,
 			  bool shareable,
 			  uint32_t *handle,
-			  struct vmw_dma_buffer **p_dma_buf)
+			  struct vmw_dma_buffer **p_dma_buf,
+			  struct ttm_base_object **p_base)
 {
 	struct vmw_user_dma_buffer *user_bo;
 	struct ttm_buffer_object *tmp;
@@ -515,6 +516,10 @@ int vmw_user_dmabuf_alloc(struct vmw_private *dev_priv,
 	}
 
 	*p_dma_buf = &user_bo->dma;
+	if (p_base) {
+		*p_base = &user_bo->prime.base;
+		kref_get(&(*p_base)->refcount);
+	}
 	*handle = user_bo->prime.base.hash.key;
 
 out_no_base_object:
@@ -631,6 +636,7 @@ int vmw_user_dmabuf_synccpu_ioctl(struct drm_device *dev, void *data,
 	struct vmw_dma_buffer *dma_buf;
 	struct vmw_user_dma_buffer *user_bo;
 	struct ttm_object_file *tfile = vmw_fpriv(file_priv)->tfile;
+	struct ttm_base_object *buffer_base;
 	int ret;
 
 	if ((arg->flags & (drm_vmw_synccpu_read | drm_vmw_synccpu_write)) == 0
@@ -643,7 +649,8 @@ int vmw_user_dmabuf_synccpu_ioctl(struct drm_device *dev, void *data,
 
 	switch (arg->op) {
 	case drm_vmw_synccpu_grab:
-		ret = vmw_user_dmabuf_lookup(tfile, arg->handle, &dma_buf);
+		ret = vmw_user_dmabuf_lookup(tfile, arg->handle, &dma_buf,
+					     &buffer_base);
 		if (unlikely(ret != 0))
 			return ret;
 
@@ -651,6 +658,7 @@ int vmw_user_dmabuf_synccpu_ioctl(struct drm_device *dev, void *data,
 				       dma);
 		ret = vmw_user_dmabuf_synccpu_grab(user_bo, tfile, arg->flags);
 		vmw_dmabuf_unreference(&dma_buf);
+		ttm_base_object_unref(&buffer_base);
 		if (unlikely(ret != 0 && ret != -ERESTARTSYS &&
 			     ret != -EBUSY)) {
 			DRM_ERROR("Failed synccpu grab on handle 0x%08x.\n",
@@ -692,7 +700,8 @@ int vmw_dmabuf_alloc_ioctl(struct drm_device *dev, void *data,
 		return ret;
 
 	ret = vmw_user_dmabuf_alloc(dev_priv, vmw_fpriv(file_priv)->tfile,
-				    req->size, false, &handle, &dma_buf);
+				    req->size, false, &handle, &dma_buf,
+				    NULL);
 	if (unlikely(ret != 0))
 		goto out_no_dmabuf;
 
@@ -721,7 +730,8 @@ int vmw_dmabuf_unref_ioctl(struct drm_device *dev, void *data,
 }
 
 int vmw_user_dmabuf_lookup(struct ttm_object_file *tfile,
-			   uint32_t handle, struct vmw_dma_buffer **out)
+			   uint32_t handle, struct vmw_dma_buffer **out,
+			   struct ttm_base_object **p_base)
 {
 	struct vmw_user_dma_buffer *vmw_user_bo;
 	struct ttm_base_object *base;
@@ -743,7 +753,10 @@ int vmw_user_dmabuf_lookup(struct ttm_object_file *tfile,
 	vmw_user_bo = container_of(base, struct vmw_user_dma_buffer,
 				   prime.base);
 	(void)ttm_bo_reference(&vmw_user_bo->dma.base);
-	ttm_base_object_unref(&base);
+	if (p_base)
+		*p_base = base;
+	else
+		ttm_base_object_unref(&base);
 	*out = &vmw_user_bo->dma;
 
 	return 0;
@@ -1004,7 +1017,7 @@ int vmw_dumb_create(struct drm_file *file_priv,
 
 	ret = vmw_user_dmabuf_alloc(dev_priv, vmw_fpriv(file_priv)->tfile,
 				    args->size, false, &args->handle,
-				    &dma_buf);
+				    &dma_buf, NULL);
 	if (unlikely(ret != 0))
 		goto out_no_dmabuf;
 
@@ -1032,7 +1045,7 @@ int vmw_dumb_map_offset(struct drm_file *file_priv,
 	struct vmw_dma_buffer *out_buf;
 	int ret;
 
-	ret = vmw_user_dmabuf_lookup(tfile, handle, &out_buf);
+	ret = vmw_user_dmabuf_lookup(tfile, handle, &out_buf, NULL);
 	if (ret != 0)
 		return -EINVAL;