[PATCH 1/2] drm/shmem-helper: Check for purged buffers in fault handler

[Neil Roberts]

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 at vger.kernel.org
Fixes: 17acb9f35ed7 ("drm/shmem: Add madvise state and purge helpers")
Signed-off-by: Neil Roberts <nroberts at igalia.com>
---
 drivers/gpu/drm/drm_gem_shmem_helper.c | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index 9825c378dfa6..b26139b1dc35 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/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)
-- 
2.29.2