Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, John Bates <jbates@chromium.org>,
        Chia-I Wu <olvaffe@gmail.com>,
        Gerd Hoffmann <kraxel@redhat.com>
Subject: [PATCH 5.5 129/189] drm/virtio: fix resource id creation race
Date:   Tue, 10 Mar 2020 13:39:26 +0100
Message-Id: <20200310123652.826628465@linuxfoundation.org>
In-Reply-To: <20200310123639.608886314@linuxfoundation.org>
References: <20200310123639.608886314@linuxfoundation.org>
User-Agent: quilt/0.66
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

From: John Bates <jbates@chromium.org>

commit fbb30168c7395b9cfeb9e6f7b0c0bca854a6552d upstream.

The previous code was not thread safe and caused
undefined behavior from spurious duplicate resource IDs.
In this patch, an atomic_t is used instead. We no longer
see any duplicate IDs in tests with this change.

Fixes: 16065fcdd19d ("drm/virtio: do NOT reuse resource ids")
Signed-off-by: John Bates <jbates@chromium.org>
Reviewed-by: Chia-I Wu <olvaffe@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200220225319.45621-1-jbates@chromium.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/gpu/drm/virtio/virtgpu_object.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/gpu/drm/virtio/virtgpu_object.c
+++ b/drivers/gpu/drm/virtio/virtgpu_object.c
@@ -42,8 +42,8 @@ static int virtio_gpu_resource_id_get(st
 		 * "f91a9dd35715 Fix unlinking resources from hash
 		 * table." (Feb 2019) fixes the bug.
 		 */
-		static int handle;
-		handle++;
+		static atomic_t seqno = ATOMIC_INIT(0);
+		int handle = atomic_inc_return(&seqno);
 		*resid = handle + 1;
 	} else {
 		int handle = ida_alloc(&vgdev->resource_ida, GFP_KERNEL);