linux-kernel - [PATCH] drm/virtio: fix undefined behavior in capset shift calculation

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Message-Id: <20251208160228.13145-1-bigalex934@gmail.com>
Date: Mon,  8 Dec 2025 19:02:29 +0300
From: Alexey Simakov <bigalex934@...il.com>
To: David Airlie <airlied@...hat.com>
Cc: Alexey Simakov <bigalex934@...il.com>,
	Gerd Hoffmann <kraxel@...hat.com>,
	Dmitry Osipenko <dmitry.osipenko@...labora.com>,
	Gurchetan Singh <gurchetansingh@...omium.org>,
	Chia-I Wu <olvaffe@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Simona Vetter <simona@...ll.ch>,
	Lingfeng Yang <lfy@...gle.com>,
	dri-devel@...ts.freedesktop.org,
	virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	lvc-project@...uxtesting.org
Subject: [PATCH] drm/virtio: fix undefined behavior in capset shift calculation

According to MAX_CAPSET_ID, possible values of vgdev->capsets[i].id
range from 0 to 63. Since the intermediate calculation uses type int,
shifting by values larger than the bit width of int is undefined
behavior as per the C language standard.

Explicitly cast the left-shift operand to ULL to prevent undefined
behavior during the calculation.

Found by Linux Verification Center (linuxtesting.org) with Svace.

Fixes: 1925d6a7e0f4 ("drm/virtio: implement context init: track valid capabilities in a mask")
Signed-off-by: Alexey Simakov <bigalex934@...il.com>
---
 drivers/gpu/drm/virtio/virtgpu_kms.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
index 1c15cbf326b7..2e0ec7590ac2 100644
--- a/drivers/gpu/drm/virtio/virtgpu_kms.c
+++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
@@ -104,7 +104,7 @@ static void virtio_gpu_get_capsets(struct virtio_gpu_device *vgdev,
 			return;
 		}
 
-		vgdev->capset_id_mask |= 1 << vgdev->capsets[i].id;
+		vgdev->capset_id_mask |= 1ULL << vgdev->capsets[i].id;
 		DRM_INFO("cap set %d: id %d, max-version %d, max-size %d\n",
 			 i, vgdev->capsets[i].id,
 			 vgdev->capsets[i].max_version,
-- 
2.34.1