drm/shmem-helper: Check for purged buffers in fault handler

When a buffer is madvised as not needed and then purged, any attempts to
access the buffer from user-space should cause a bus fault. This patch
adds a check for that.

Cc: stable@vger.kernel.org
Fixes: 17acb9f3 ("drm/shmem: Add madvise state and purge helpers")
Signed-off-by: Neil Roberts <nroberts@igalia.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210223155125.199577-2-nroberts@igalia.comSigned-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -525,14 +525,24 @@ static vm_fault_t drm_gem_shmem_fault(struct vm_fault *vmf)
 	struct drm_gem_object *obj = vma->vm_private_data;
 	struct drm_gem_shmem_object *shmem = to_drm_gem_shmem_obj(obj);
 	loff_t num_pages = obj->size >> PAGE_SHIFT;
+	vm_fault_t ret;
 	struct page *page;
 
-	if (vmf->pgoff >= num_pages || WARN_ON_ONCE(!shmem->pages))
-		return VM_FAULT_SIGBUS;
+	mutex_lock(&shmem->pages_lock);
+
+	if (vmf->pgoff >= num_pages ||
+	    WARN_ON_ONCE(!shmem->pages) ||
+	    shmem->madv < 0) {
+		ret = VM_FAULT_SIGBUS;
+	} else {
+		page = shmem->pages[vmf->pgoff];
 
-	page = shmem->pages[vmf->pgoff];
+		ret = vmf_insert_page(vma, vmf->address, page);
+	}
 
-	return vmf_insert_page(vma, vmf->address, page);
+	mutex_unlock(&shmem->pages_lock);
+
+	return ret;
 }
 
 static void drm_gem_shmem_vm_open(struct vm_area_struct *vma)