Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Rob Clark <robdclark@gmail.com>
To:     dri-devel@lists.freedesktop.org
Cc:     Rob Clark <robdclark@chromium.org>,
        David Airlie <airlied@redhat.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Gurchetan Singh <gurchetansingh@chromium.org>,
        Chia-I Wu <olvaffe@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
        virtualization@lists.linux-foundation.org (open list:VIRTIO GPU DRIVER),
        linux-kernel@vger.kernel.org (open list)
Subject: [PATCH] drm/virtio: Fix GEM handle creation UAF
Date:   Fri, 16 Dec 2022 15:33:55 -0800
Message-Id: <20221216233355.542197-2-robdclark@gmail.com>
In-Reply-To: <20221216233355.542197-1-robdclark@gmail.com>
References: <20221216233355.542197-1-robdclark@gmail.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

From: Rob Clark <robdclark@chromium.org>

Userspace can guess the handle value and try to race GEM object creation
with handle close, resulting in a use-after-free if we dereference the
object after dropping the handle's reference.  For that reason, dropping
the handle's reference must be done *after* we are done dereferencing
the object.

Signed-off-by: Rob Clark <robdclark@chromium.org>
---
 drivers/gpu/drm/virtio/virtgpu_ioctl.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_ioctl.c b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
index a5cccccb4998..f1c55c1630ca 100644
--- a/drivers/gpu/drm/virtio/virtgpu_ioctl.c
+++ b/drivers/gpu/drm/virtio/virtgpu_ioctl.c
@@ -366,10 +366,18 @@ static int virtio_gpu_resource_create_ioctl(struct drm_device *dev, void *data,
 		drm_gem_object_release(obj);
 		return ret;
 	}
-	drm_gem_object_put(obj);
 
 	rc->res_handle = qobj->hw_res_handle; /* similiar to a VM address */
 	rc->bo_handle = handle;
+
+	/*
+	 * The handle owns the reference now.  But we must drop our
+	 * remaining reference *after* we no longer need to dereference
+	 * the obj.  Otherwise userspace could guess the handle and
+	 * race closing it from another thread.
+	 */
+	drm_gem_object_put(obj);
+
 	return 0;
 }
 
@@ -731,11 +739,18 @@ static int virtio_gpu_resource_create_blob_ioctl(struct drm_device *dev,
 		drm_gem_object_release(obj);
 		return ret;
 	}
-	drm_gem_object_put(obj);
 
 	rc_blob->res_handle = bo->hw_res_handle;
 	rc_blob->bo_handle = handle;
 
+	/*
+	 * The handle owns the reference now.  But we must drop our
+	 * remaining reference *after* we no longer need to dereference
+	 * the obj.  Otherwise userspace could guess the handle and
+	 * race closing it from another thread.
+	 */
+	drm_gem_object_put(obj);
+
 	return 0;
 }
 
-- 
2.38.1