[RFC PATCH] drm/ttm: Fix swapping dereferences of freed memory

[Thomas Hellström]

The swapping code was dereference bo->ttm pointers without having the
dma-resv lock held. Also it might try to swap out unpopulated bos.

Fix this by moving the bo->ttm dereference until we have the reservation
lock. Check that the ttm_tt is populated after the swap_notify callback.

Signed-off-by: Thomas Hellström <thomas.hellstrom at linux.intel.com>
---
 drivers/gpu/drm/ttm/ttm_bo.c     | 16 +++++++++++++++-
 drivers/gpu/drm/ttm/ttm_device.c |  8 +++-----
 2 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
index 9f53506a82fc..86213d37657b 100644
--- a/drivers/gpu/drm/ttm/ttm_bo.c
+++ b/drivers/gpu/drm/ttm/ttm_bo.c
@@ -1163,6 +1163,16 @@ int ttm_bo_swapout(struct ttm_buffer_object *bo, struct ttm_operation_ctx *ctx,
 	if (!ttm_bo_evict_swapout_allowable(bo, ctx, &place, &locked, NULL))
 		return -EBUSY;
 
+	dma_resv_assert_held(bo->base.resv);
+
+	if (!bo->ttm ||
+	    bo->ttm->page_flags & TTM_PAGE_FLAG_SG ||
+	    bo->ttm->page_flags & TTM_PAGE_FLAG_SWAPPED) {
+		if (locked)
+			dma_resv_unlock(bo->base.resv);
+		return -EBUSY;
+	}
+
 	if (!ttm_bo_get_unless_zero(bo)) {
 		if (locked)
 			dma_resv_unlock(bo->base.resv);
@@ -1215,7 +1225,8 @@ int ttm_bo_swapout(struct ttm_buffer_object *bo, struct ttm_operation_ctx *ctx,
 	if (bo->bdev->funcs->swap_notify)
 		bo->bdev->funcs->swap_notify(bo);
 
-	ret = ttm_tt_swapout(bo->bdev, bo->ttm, gfp_flags);
+	if (ttm_tt_is_populated(bo->ttm))
+		ret = ttm_tt_swapout(bo->bdev, bo->ttm, gfp_flags);
 out:
 
 	/*
@@ -1225,6 +1236,9 @@ int ttm_bo_swapout(struct ttm_buffer_object *bo, struct ttm_operation_ctx *ctx,
 	if (locked)
 		dma_resv_unlock(bo->base.resv);
 	ttm_bo_put(bo);
+
+	/* Don't break locking rules. */
+	WARN_ON(ret == -EBUSY);
 	return ret;
 }
 
diff --git a/drivers/gpu/drm/ttm/ttm_device.c b/drivers/gpu/drm/ttm/ttm_device.c
index 460953dcad11..eaa7487ae404 100644
--- a/drivers/gpu/drm/ttm/ttm_device.c
+++ b/drivers/gpu/drm/ttm/ttm_device.c
@@ -143,14 +143,12 @@ int ttm_device_swapout(struct ttm_device *bdev, struct ttm_operation_ctx *ctx,
 
 		for (j = 0; j < TTM_MAX_BO_PRIORITY; ++j) {
 			list_for_each_entry(bo, &man->lru[j], lru) {
-				uint32_t num_pages;
+				pgoff_t num_pages;
 
-				if (!bo->ttm ||
-				    bo->ttm->page_flags & TTM_PAGE_FLAG_SG ||
-				    bo->ttm->page_flags & TTM_PAGE_FLAG_SWAPPED)
+				if (!READ_ONCE(bo->ttm))
 					continue;
 
-				num_pages = bo->ttm->num_pages;
+				num_pages = bo->base.size >> PAGE_SHIFT;
 				ret = ttm_bo_swapout(bo, ctx, gfp_flags);
 				/* ttm_bo_swapout has dropped the lru_lock */
 				if (!ret)
-- 
2.31.1