Re: [Qemu-devel] [PATCH V2] virtio: properly validate address before accessing config

[Petr Matousek]

On Tue, May 07, 2013 at 01:42:49PM +0800, Jason Wang wrote:
> There are several several issues in the current checking:
> 
> - The check was based on the minus of unsigned values which can overflow
> - It was done after .{set|get}_config() which can lead crash when config_len
>   is zero since vdev->config is NULL
> 
> Fix this by:
> 
> - Validate the address in virtio_pci_config_{read|write}() before
>   .{set|get}_config
> - Use addition instead minus to do the validation
> 
> Cc: Michael S. Tsirkin <address@hidden>
> Cc: Petr Matousek <address@hidden>
> Signed-off-by: Jason Wang <address@hidden>

I would rather see explicit check for (addr + sizeof(val)) overflow, but
I was able to convince myself that it cannot happen due to the addr
properties.

Acked-by: Petr Matousek <address@hidden>

> 
> ---
> Changes from V1:
> - Doing check in virtio.c instead of virtio-pci.c
> - Drop the patch of virtio-ccw and s390-virtio-bus
> ---
>  hw/virtio/virtio.c |   30 ++++++++++++++++++------------
>  1 files changed, 18 insertions(+), 12 deletions(-)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 53a0d90..8176c14 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -568,10 +568,11 @@ uint32_t virtio_config_readb(VirtIODevice *vdev, 
> uint32_t addr)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      uint8_t val;
>  
> -    k->get_config(vdev, vdev->config);
> -
> -    if (addr > (vdev->config_len - sizeof(val)))
> +    if (addr + sizeof(val) > vdev->config_len) {
>          return (uint32_t)-1;
> +    }
> +
> +    k->get_config(vdev, vdev->config);
>  
>      val = ldub_p(vdev->config + addr);
>      return val;
> @@ -582,10 +583,11 @@ uint32_t virtio_config_readw(VirtIODevice *vdev, 
> uint32_t addr)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      uint16_t val;
>  
> -    k->get_config(vdev, vdev->config);
> -
> -    if (addr > (vdev->config_len - sizeof(val)))
> +    if (addr + sizeof(val) > vdev->config_len) {
>          return (uint32_t)-1;
> +    }
> +
> +    k->get_config(vdev, vdev->config);
>  
>      val = lduw_p(vdev->config + addr);
>      return val;
> @@ -596,10 +598,11 @@ uint32_t virtio_config_readl(VirtIODevice *vdev, 
> uint32_t addr)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      uint32_t val;
>  
> -    k->get_config(vdev, vdev->config);
> -
> -    if (addr > (vdev->config_len - sizeof(val)))
> +    if (addr + sizeof(val) > vdev->config_len) {
>          return (uint32_t)-1;
> +    }
> +
> +    k->get_config(vdev, vdev->config);
>  
>      val = ldl_p(vdev->config + addr);
>      return val;
> @@ -610,8 +613,9 @@ void virtio_config_writeb(VirtIODevice *vdev, uint32_t 
> addr, uint32_t data)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      uint8_t val = data;
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> +    if (addr + sizeof(val) > vdev->config_len) {
>          return;
> +    }
>  
>      stb_p(vdev->config + addr, val);
>  
> @@ -625,8 +629,9 @@ void virtio_config_writew(VirtIODevice *vdev, uint32_t 
> addr, uint32_t data)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      uint16_t val = data;
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> +    if (addr + sizeof(val) > vdev->config_len) {
>          return;
> +    }
>  
>      stw_p(vdev->config + addr, val);
>  
> @@ -640,8 +645,9 @@ void virtio_config_writel(VirtIODevice *vdev, uint32_t 
> addr, uint32_t data)
>      VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(vdev);
>      uint32_t val = data;
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> +    if (addr + sizeof(val) > vdev->config_len) {
>          return;
> +    }
>  
>      stl_p(vdev->config + addr, val);
>  
> -- 
> 1.7.1
> 

-- 
Petr Matousek / Red Hat Security Response Team