[PATCH v2 2/2] drm/xe: Fix stale pinned_link entry when fb-pin performs the final unpin

["Thomas Hellström" <thomas.hellstrom@linux.intel.com>]

xe_bo_pin_external() and xe_bo_unpin_external() maintain the bo's
pinned_link list membership in xe->pinned.late.external, based on
whether the current call is the outermost pin or the final unpin.
However, the same external bo's pin_count can also be raised and
lowered directly by __xe_pin_fb_vma()/__xe_unpin_fb_vma(), which pin
the bo as a display scanout buffer without going through
xe_bo_pin_external()/xe_bo_unpin_external() at all, and have no
notion of, or ownership over, pinned_link.

If a bo is pinned both externally (e.g. dma-buf export) and as an fb,
and the external unpin happens first, xe_bo_unpin_external() correctly
observes pin_count > 1 and leaves the bo on pinned_link. When the fb
unpin later performs the true last unpin (pin_count 1 -> 0), it never
touches pinned_link, leaving the bo linked on xe->pinned.late.external
indefinitely. Once the bo is subsequently freed, this stale list entry
points into freed memory, corrupting the list and risking a
use-after-free the next time the list is walked or spliced.

The backup object pin/unpin sites in xe_bo_notifier_prepare_pinned()/
xe_bo_notifier_unprepare_pinned() have the same bypass characteristic,
though they never add their bo to a pinned list, so are not affected
by this particular list-corruption issue.

Move the pinned_link removal into xe_bo_account_unpin(), which already
runs on every unpin path (kernel, external, framebuffer, backup
object) right before the true 1 -> 0 pin_count transition. Since
list_del_init() only operates on the node itself, this removal is
list-agnostic and safe to perform regardless of which list (external
or kernel_bo_present) the bo happens to be linked on, or which code
path is performing the final unpin. Drop the now-redundant explicit
list_del_init() calls in xe_bo_unpin_external() and xe_bo_unpin().

Fixes: 44e694958b95 ("drm/xe/display: Implement display support")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: intel-xe@lists.freedesktop.org
Cc: <stable@vger.kernel.org> # v6.8+
Assisted-by: LLM
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

v2:
- New patch
---
 drivers/gpu/drm/xe/xe_bo.c | 25 ++++++++++++++++---------
 1 file changed, 16 insertions(+), 9 deletions(-)

diff --git a/drivers/gpu/drm/xe/xe_bo.c b/drivers/gpu/drm/xe/xe_bo.c
index 2fbbba7cf4b0..581bfded21db 100644
--- a/drivers/gpu/drm/xe/xe_bo.c
+++ b/drivers/gpu/drm/xe/xe_bo.c
@@ -487,12 +487,27 @@ static void xe_bo_account_pin(struct xe_bo *bo)
  * held, before &ttm_buffer_object.pin_count is decremented by
  * ttm_bo_unpin(), so that the check against the true 1->0 transition sees
  * the pin count that is about to be released.
+ *
+ * On the true last unpin, also removes @bo from whichever pinned-bo list
+ * (external or kernel_bo_present) it may currently be linked on, since a
+ * bo's final unpin can happen through a pin path (e.g. framebuffer,
+ * backup object) that has no notion of, or ownership over, that list.
+ * This is safe and list-agnostic: list_del_init() only needs the node
+ * itself, not knowledge of which list it is threaded through, and is a
+ * no-op if @bo is not linked.
  */
 static void xe_bo_account_unpin(struct xe_bo *bo)
 {
 	struct xe_device *xe = xe_bo_device(bo);
+	bool last_unpin = bo->ttm.pin_count == 1;
 
-	if (bo->ttm.pin_count == 1 && bo->ttm.ttm && ttm_tt_is_populated(bo->ttm.ttm))
+	if (last_unpin && !list_empty(&bo->pinned_link)) {
+		spin_lock(&xe->pinned.lock);
+		list_del_init(&bo->pinned_link);
+		spin_unlock(&xe->pinned.lock);
+	}
+
+	if (last_unpin && bo->ttm.ttm && ttm_tt_is_populated(bo->ttm.ttm))
 		xe_ttm_tt_account_add(xe, bo->ttm.ttm);
 }
 
@@ -3311,11 +3326,6 @@ void xe_bo_unpin_external(struct xe_bo *bo)
 	xe_assert(xe, xe_bo_is_pinned(bo));
 	xe_assert(xe, xe_bo_is_user(bo));
 
-	spin_lock(&xe->pinned.lock);
-	if (bo->ttm.pin_count == 1 && !list_empty(&bo->pinned_link))
-		list_del_init(&bo->pinned_link);
-	spin_unlock(&xe->pinned.lock);
-
 	xe_bo_unpin_account(bo);
 
 	/*
@@ -3337,10 +3347,7 @@ void xe_bo_unpin(struct xe_bo *bo)
 	xe_assert(xe, xe_bo_is_pinned(bo));
 
 	if (mem_type_is_vram(place->mem_type) || bo->flags & XE_BO_FLAG_GGTT) {
-		spin_lock(&xe->pinned.lock);
 		xe_assert(xe, !list_empty(&bo->pinned_link));
-		list_del_init(&bo->pinned_link);
-		spin_unlock(&xe->pinned.lock);
 
 		if (bo->backup_obj) {
 			if (xe_bo_is_pinned(bo->backup_obj))
-- 
2.55.0