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

[Markus Armbruster]

Alex Williamson <address@hidden> writes:

> 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,

Fair point.  v2 sent.