drm/crtc: Fix uninit-value bug in drm_mode_setcrtc

The connector_set contains uninitialized values when allocated with
kmalloc_array. However, in the "out" branch, the logic assumes that any
element in connector_set would be equal to NULL if failed to
initialize, which causes the bug reported by Syzbot. The fix is to use
an extra variable to keep track of how many connectors are initialized
indeed, and use that variable to decrease any refcounts in the "out"
branch.

Reported-by: [email protected]
Signed-off-by: Ziqi Zhao <[email protected]>
Reported-and-tested-by: [email protected]
Tested-by: Harshit Mogalapalli <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Maxime Ripard <[email protected]>

drivers/gpu/drm/drm_crtc.c

@@ -715,8 +715,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 	struct drm_mode_set set;
 	uint32_t __user *set_connectors_ptr;
 	struct drm_modeset_acquire_ctx ctx;
-	int ret;
-	int i;
+	int ret, i, num_connectors;
 
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return -EOPNOTSUPP;
@@ -851,6 +850,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 			goto out;
 		}
 
+		num_connectors = 0;
 		for (i = 0; i < crtc_req->count_connectors; i++) {
 			connector_set[i] = NULL;
 			set_connectors_ptr = (uint32_t __user *)(unsigned long)crtc_req->set_connectors_ptr;
@@ -871,6 +871,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 					connector->name);
 
 			connector_set[i] = connector;
+			num_connectors++;
 		}
 	}
 
@@ -879,7 +880,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 	set.y = crtc_req->y;
 	set.mode = mode;
 	set.connectors = connector_set;
-	set.num_connectors = crtc_req->count_connectors;
+	set.num_connectors = num_connectors;
 	set.fb = fb;
 
 	if (drm_drv_uses_atomic_modeset(dev))
@@ -892,7 +893,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 		drm_framebuffer_put(fb);
 
 	if (connector_set) {
-		for (i = 0; i < crtc_req->count_connectors; i++) {
+		for (i = 0; i < num_connectors; i++) {
 			if (connector_set[i])
 				drm_connector_put(connector_set[i]);
 		}