[PATCH v2 2/2] drm/modeset: harden modeset_lock() against NULL ctx

[george.d.sworo@intel.com]

From: George D Sworo <george.d.sworo@intel.com>

modeset_lock() dereferences ctx unconditionally. Add a defensive NULL
guard to avoid NULL dereference if a buggy internal caller passes a NULL
acquire context.

For NULL ctx, fall back to plain ww_mutex locking semantics:
- interruptible path uses ww_mutex_lock_interruptible(..., NULL)
- non-interruptible path uses ww_mutex_lock(..., NULL)

This keeps wait behavior consistent with the helper arguments.

Signed-off-by: George D Sworo <george.d.sworo@intel.com>
---
 drivers/gpu/drm/drm_modeset_lock.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/drivers/gpu/drm/drm_modeset_lock.c b/drivers/gpu/drm/drm_modeset_lock.c
index 2052bb9bb9e5..5bee424805c3 100644
--- a/drivers/gpu/drm/drm_modeset_lock.c
+++ b/drivers/gpu/drm/drm_modeset_lock.c
@@ -294,6 +294,18 @@ static inline int modeset_lock(struct drm_modeset_lock *lock,
 		bool interruptible, bool slow)
 {
 	int ret;
+	/*
+	 * Defensive fallback: this helper is expected to be called with a
+	 * valid acquire context, but if a NULL ctx slips through, preserve
+	 * the lock wait semantics and avoid NULL dereference.
+	 */
+	if (unlikely(!ctx)) {
+		if (interruptible)
+			return ww_mutex_lock_interruptible(&lock->mutex, NULL);
+
+		ww_mutex_lock(&lock->mutex, NULL);
+		return 0;
+	}
 
 	if (ctx && WARN_ON(ctx->contended))
 		__drm_stack_depot_print(ctx->stack_depot);
-- 
2.34.1