Re: [Qemu-devel] [PATCH 1/2] vfio: Fix overrun after readlink() fills buffer completely

[Alex Williamson]

On Fri, 2014-02-21 at 17:42 +0100, Markus Armbruster wrote:
> readlink() returns the number of bytes written to the buffer, and it
> doesn't write a terminating null byte.  vfio_init() writes it itself.
> Overruns the buffer when readlink() filled it completely.
> 
> Fix by reserving space for the null byte when calling readlink(), like
> we do elsewhere.
> 
> Spotted by Coverity.
> 
> Signed-off-by: Markus Armbruster <address@hidden>
> ---
>  hw/misc/vfio.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/misc/vfio.c b/hw/misc/vfio.c
> index 8db182f..8e56785 100644
> --- a/hw/misc/vfio.c
> +++ b/hw/misc/vfio.c
> @@ -3681,7 +3681,7 @@ static int vfio_initfn(PCIDevice *pdev)
>  
>      strncat(path, "iommu_group", sizeof(path) - strlen(path) - 1);
>  
> -    len = readlink(path, iommu_group_path, PATH_MAX);
> +    len = readlink(path, iommu_group_path, PATH_MAX - 1);
>      if (len <= 0) {
>          error_report("vfio: error no iommu_group for device");
>          return -errno;

I'm not sure why we wouldn't follow the same logic as pci-assign here.
If we fill to the length provided we have no idea if we have the full
path.  We certainly expect it to fit well within PATH_MAX, so let's
handle it as an error if we reach PATH_MAX.  Thanks,

Alex