Re: [PATCH v1 2/6] virtio console: Harden port adding

Michael S. Tsirkin
2023-01-27T11:03:39Z

On Thu, Jan 19, 2023 at 10:13:18PM +0200, Alexander Shishkin wrote:
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>
> > Then you need to copy it out once, and then only deal with the local
> > copy. Otherwise you have an incomplete snapshot.
>
> Ok, would you be partial to something like this:
>
> >From 1bc9bb84004154376c2a0cf643d53257da6d1cd7 Mon Sep 17 00:00:00 2001
> From: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Date: Thu, 19 Jan 2023 21:59:02 +0200
> Subject: [PATCH] virtio console: Keep a local copy of the control structure
>
> When handling control messages, instead of peeking at the device memory
> to obtain bits of the control structure,

Except the message makes it seem that we are getting data from
device memory, when we do nothing of the kind.

> take a snapshot of it once and
> use it instead, to prevent it from changing under us. This avoids races
> between port id validation and control event decoding, which can lead
> to, for example, a NULL dereference in port removal of a nonexistent
> port.
>
> The control structure is small enough (8 bytes) that it can be cached
> directly on the stack.

I still have no real idea why we want a copy here.
If device can poke anywhere at memory then it can crash kernel anyway.
If there's a bounce buffer or an iommu or some other protection
in place, then this memory can no longer change by the time
we look at it.

> Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Amit Shah <amit@kernel.org>
> ---
> drivers/char/virtio_console.c | 29 +++++++++++++++--------------
> 1 file changed, 15 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/char/virtio_console.c b/drivers/char/virtio_console.c
> index 6a821118d553..42be0991a72f 100644
> --- a/drivers/char/virtio_console.c
> +++ b/drivers/char/virtio_console.c
> @@ -1559,23 +1559,24 @@ static void handle_control_message(struct virtio_device *vdev,
> struct ports_device *portdev,
> struct port_buffer *buf)
> {
> - struct virtio_console_control *cpkt;
> + struct virtio_console_control cpkt;
> struct port *port;
> size_t name_size;
> int err;
>
> - cpkt = (struct virtio_console_control *)(buf->buf + buf->offset);
> + /* Keep a local copy of the control structure */
> + memcpy(&cpkt, buf->buf + buf->offset, sizeof(cpkt));
>
> - port = find_port_by_id(portdev, virtio32_to_cpu(vdev, cpkt->id));
> + port = find_port_by_id(portdev, virtio32_to_cpu(vdev, cpkt.id));
> if (!port &&
> - cpkt->event != cpu_to_virtio16(vdev, VIRTIO_CONSOLE_PORT_ADD)) {
> + cpkt.event != cpu_to_virtio16(vdev, VIRTIO_CONSOLE_PORT_ADD)) {
> /* No valid header at start of buffer. Drop it. */
> dev_dbg(&portdev->vdev->dev,
> - "Invalid index %u in control packet\n", cpkt->id);
> + "Invalid index %u in control packet\n", cpkt.id);
> return;
> }
>
> - switch (virtio16_to_cpu(vdev, cpkt->event)) {
> + switch (virtio16_to_cpu(vdev, cpkt.event)) {
> case VIRTIO_CONSOLE_PORT_ADD:
> if (port) {
> dev_dbg(&portdev->vdev->dev,
> @@ -1583,21 +1584,21 @@ static void handle_control_message(struct virtio_device *vdev,
> send_control_msg(port, VIRTIO_CONSOLE_PORT_READY, 1);
> break;
> }
> - if (virtio32_to_cpu(vdev, cpkt->id) >=
> + if (virtio32_to_cpu(vdev, cpkt.id) >=
> portdev->max_nr_ports) {
> dev_warn(&portdev->vdev->dev,
> "Request for adding port with "
> "out-of-bound id %u, max. supported id: %u\n",
> - cpkt->id, portdev->max_nr_ports - 1);
> + cpkt.id, portdev->max_nr_ports - 1);
> break;
> }
> - add_port(portdev, virtio32_to_cpu(vdev, cpkt->id));
> + add_port(portdev, virtio32_to_cpu(vdev, cpkt.id));
> break;
> case VIRTIO_CONSOLE_PORT_REMOVE:
> unplug_port(port);
> break;
> case VIRTIO_CONSOLE_CONSOLE_PORT:
> - if (!cpkt->value)
> + if (!cpkt.value)
> break;
> if (is_console_port(port))
> break;
> @@ -1618,7 +1619,7 @@ static void handle_control_message(struct virtio_device *vdev,
> if (!is_console_port(port))
> break;
>
> - memcpy(&size, buf->buf + buf->offset + sizeof(*cpkt),
> + memcpy(&size, buf->buf + buf->offset + sizeof(cpkt),
> sizeof(size));
> set_console_size(port, size.rows, size.cols);
>
> @@ -1627,7 +1628,7 @@ static void handle_control_message(struct virtio_device *vdev,
> break;
> }
> case VIRTIO_CONSOLE_PORT_OPEN:
> - port->host_connected = virtio16_to_cpu(vdev, cpkt->value);
> + port->host_connected = virtio16_to_cpu(vdev, cpkt.value);
> wake_up_interruptible(&port->waitqueue);
> /*
> * If the host port got closed and the host had any
> @@ -1658,7 +1659,7 @@ static void handle_control_message(struct virtio_device *vdev,
> * Skip the size of the header and the cpkt to get the size
> * of the name that was sent
> */
> - name_size = buf->len - buf->offset - sizeof(*cpkt) + 1;
> + name_size = buf->len - buf->offset - sizeof(cpkt) + 1;
>
> port->name = kmalloc(name_size, GFP_KERNEL);
> if (!port->name) {
> @@ -1666,7 +1667,7 @@ static void handle_control_message(struct virtio_device *vdev,
> "Not enough space to store port name\n");
> break;
> }
> - strncpy(port->name, buf->buf + buf->offset + sizeof(*cpkt),
> + strncpy(port->name, buf->buf + buf->offset + sizeof(cpkt),
> name_size - 1);
> port->name[name_size - 1] = 0;
>
> --
> 2.39.0