Re: [Qemu-devel] [PATCH 1/3] virtio-pci: properly validate address before accessing config

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Apr 26, 2013 at 04:34:02PM +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 <mst@redhat.com>
> Cc: Petr Matousek <pmatouse@redhat.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>

Why do this in virtio-pci and not in virtio.c?
If instead we correct the checks in virtio.c we
get less code, and all transports will benefit
automatically.

> ---
>  hw/virtio/virtio-pci.c |    9 +++++++++
>  hw/virtio/virtio.c     |   18 ------------------
>  2 files changed, 9 insertions(+), 18 deletions(-)
> 
> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c
> index a1f15a8..7f6c7d1 100644
> --- a/hw/virtio/virtio-pci.c
> +++ b/hw/virtio/virtio-pci.c
> @@ -400,6 +400,10 @@ static uint64_t virtio_pci_config_read(void *opaque, hwaddr addr,
>      }
>      addr -= config;
>  
> +    if (addr + size > proxy->vdev->config_len) {
> +        return (uint32_t)-1;
> +    }
> +
>      switch (size) {
>      case 1:
>          val = virtio_config_readb(proxy->vdev, addr);
> @@ -430,6 +434,11 @@ static void virtio_pci_config_write(void *opaque, hwaddr addr,
>          return;
>      }
>      addr -= config;
> +
> +    if (addr + size > proxy->vdev->config_len) {
> +        return;
> +    }
> +
>      /*
>       * Virtio-PCI is odd. Ioports are LE but config space is target native
>       * endian.
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 1c2282c..3397b5e 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -563,9 +563,6 @@ uint32_t virtio_config_readb(VirtIODevice *vdev, uint32_t addr)
>  
>      vdev->get_config(vdev, vdev->config);
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> -        return (uint32_t)-1;
> -
>      val = ldub_p(vdev->config + addr);
>      return val;
>  }
> @@ -576,9 +573,6 @@ uint32_t virtio_config_readw(VirtIODevice *vdev, uint32_t addr)
>  
>      vdev->get_config(vdev, vdev->config);
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> -        return (uint32_t)-1;
> -
>      val = lduw_p(vdev->config + addr);
>      return val;
>  }
> @@ -589,9 +583,6 @@ uint32_t virtio_config_readl(VirtIODevice *vdev, uint32_t addr)
>  
>      vdev->get_config(vdev, vdev->config);
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> -        return (uint32_t)-1;
> -
>      val = ldl_p(vdev->config + addr);
>      return val;
>  }
> @@ -600,9 +591,6 @@ void virtio_config_writeb(VirtIODevice *vdev, uint32_t addr, uint32_t data)
>  {
>      uint8_t val = data;
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> -        return;
> -
>      stb_p(vdev->config + addr, val);
>  
>      if (vdev->set_config)
> @@ -613,9 +601,6 @@ void virtio_config_writew(VirtIODevice *vdev, uint32_t addr, uint32_t data)
>  {
>      uint16_t val = data;
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> -        return;
> -
>      stw_p(vdev->config + addr, val);
>  
>      if (vdev->set_config)
> @@ -626,9 +611,6 @@ void virtio_config_writel(VirtIODevice *vdev, uint32_t addr, uint32_t data)
>  {
>      uint32_t val = data;
>  
> -    if (addr > (vdev->config_len - sizeof(val)))
> -        return;
> -
>      stl_p(vdev->config + addr, val);
>  
>      if (vdev->set_config)
> -- 
> 1.7.1