[PATCH v2] drm/msm: Fix use-after-free in msm_gem with carveout

[Iskren Chernev]

When using gem with vram carveout the page allocation is managed via
drm_mm. The necessary drm_mm_node is allocated in add_vma, but it is
referenced in msm_gem_object as well. It is freed before the drm_mm_node
has been deallocated leading to use-after-free on every single vram
allocation.

Currently put_iova is called before put_pages in both
msm_gem_free_object and msm_gem_purge:

	put_iova -> del_vma -> kfree(vma) // vma holds drm_mm_node
	/* later */
	put_pages -> put_pages_vram -> drm_mm_remove_node(
						msm_obj->vram_node)
				 	// vram_node is a ref to
					// drm_mm_node; in _msm_gem_new

It looks like del_vma does nothing else other than freeing the vma
object and removing it from it's list, so delaying the deletion should
be harmless.

This patch splits put_iova in put_iova_spaces and put_iova_vmas, so the
vma can be freed after the mm_node has been deallocated with the mm.

Note: The breaking commit separated the vma allocation from within
msm_gem_object to outside, so the vram_node reference became outside the
msm_gem_object allocation, and freeing order was therefore overlooked.

Fixes: 4b85f7f5cf7 ("drm/msm: support for an arbitrary number of address spaces")
Signed-off-by: Iskren Chernev <iskren.chernev@xxxxxxxxx>
---
v1: https://lkml.org/lkml/2020/11/26/130

Changes in v2:
- patch now compiles (oops)
- improve commit message
- add fixes tag

 drivers/gpu/drm/msm/msm_gem.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/msm/msm_gem.c b/drivers/gpu/drm/msm/msm_gem.c
index 15715a156620f..dfe6387c62c86 100644
--- a/drivers/gpu/drm/msm/msm_gem.c
+++ b/drivers/gpu/drm/msm/msm_gem.c
@@ -355,18 +355,31 @@ static void del_vma(struct msm_gem_vma *vma)

 /* Called with msm_obj locked */
 static void
-put_iova(struct drm_gem_object *obj)
+put_iova_spaces(struct drm_gem_object *obj)
 {
 	struct msm_gem_object *msm_obj = to_msm_bo(obj);
-	struct msm_gem_vma *vma, *tmp;
+	struct msm_gem_vma *vma;

 	WARN_ON(!msm_gem_is_locked(obj));

-	list_for_each_entry_safe(vma, tmp, &msm_obj->vmas, list) {
+	list_for_each_entry(vma, &msm_obj->vmas, list) {
 		if (vma->aspace) {
 			msm_gem_purge_vma(vma->aspace, vma);
 			msm_gem_close_vma(vma->aspace, vma);
 		}
+	}
+}
+
+/* Called with msm_obj locked */
+static void
+put_iova_vmas(struct drm_gem_object *obj)
+{
+	struct msm_gem_object *msm_obj = to_msm_bo(obj);
+	struct msm_gem_vma *vma, *tmp;
+
+	WARN_ON(!msm_gem_is_locked(obj));
+
+	list_for_each_entry_safe(vma, tmp, &msm_obj->vmas, list) {
 		del_vma(vma);
 	}
 }
@@ -688,12 +701,14 @@ void msm_gem_purge(struct drm_gem_object *obj)
 	WARN_ON(!is_purgeable(msm_obj));
 	WARN_ON(obj->import_attach);

-	put_iova(obj);
+	put_iova_spaces(obj);

 	msm_gem_vunmap(obj);

 	put_pages(obj);

+	put_iova_vmas(obj);
+
 	msm_obj->madv = __MSM_MADV_PURGED;

 	drm_vma_node_unmap(&obj->vma_node, dev->anon_inode->i_mapping);
@@ -942,7 +957,7 @@ void msm_gem_free_object(struct drm_gem_object *obj)

 	msm_gem_lock(obj);

-	put_iova(obj);
+	put_iova_spaces(obj);

 	if (obj->import_attach) {
 		WARN_ON(msm_obj->vaddr);
@@ -965,6 +980,8 @@ void msm_gem_free_object(struct drm_gem_object *obj)
 		msm_gem_unlock(obj);
 	}

+	put_iova_vmas(obj);
+
 	drm_gem_object_release(obj);

 	kfree(msm_obj);

base-commit: 6147c83fd749d19a0d3ccc2f64d12138ab010b47
--
2.29.2