Re: [PATCH v3 08/17] iommufd: IOMMU_HWPT_ALLOC allocation with user data

[Jason Gunthorpe <jgg@nvidia.com>]

On Wed, Aug 02, 2023 at 05:53:40PM -0700, Nicolin Chen wrote:
> On Wed, Aug 02, 2023 at 08:43:12PM -0300, Jason Gunthorpe wrote:
> > On Wed, Aug 02, 2023 at 04:42:10PM -0700, Nicolin Chen wrote:
> > > On Mon, Jul 31, 2023 at 10:16:17AM -0300, Jason Gunthorpe wrote:
> > >  
> > > > > Ideally expanding uAPI structure size should come with new flag bits.
> > > > 
> > > > Flags or some kind of 'zero is the same behavior as a smaller struct'
> > > > scheme.
> > > > 
> > > > This patch is doing the zero option:
> > > > 
> > > >  	__u32 __reserved;
> > > > +	__u32 hwpt_type;
> > > > +	__u32 data_len;
> > > > +	__aligned_u64 data_uptr;
> > > >  };
> > > > 
> > > > hwpt_type == 0 means default type
> > > > data_len == 0 means no data
> > > > data_uptr is ignored (zero is safe)
> > > > 
> > > > So there is no need to change it
> > > 
> > > TEST_LENGTH passing ".size = sizeof(struct _struct) - 1" expects a
> > > -EINVAL error code from "if (ucmd.user_size < op->min_size)" check
> > > in the iommufd_fops_ioctl(). This has been working when min_size is
> > > exactly the size of the structure.
> > > 
> > > When the size of the structure becomes larger than min_size, i.e.
> > > the passing size above is larger than min_size, it bypasses that
> > > min_size sanity and goes down to an ioctl handler with a potential
> > > risk. And actually, the size range can be [min_size, struct_size),
> > > making it harder for us to sanitize with the existing code.
> > > 
> > > I wonder what's the generic way of sanitizing this case? And, it
> > > seems that TEST_LENGTH needs some rework to test min_size only?
> > 
> > Yes, it should technically test using offsetof and a matching set of
> > struct members.
> 
> OK. I copied 3 lines for offsetofend from the kernel and did this:
> --------------------------------------------------------------------------
> diff --git a/tools/testing/selftests/iommu/iommufd.c b/tools/testing/selftests/iommu/iommufd.c
> index 6b075a68b928..a15a475c1243 100644
> --- a/tools/testing/selftests/iommu/iommufd.c
> +++ b/tools/testing/selftests/iommu/iommufd.c
> @@ -86,12 +86,13 @@ TEST_F(iommufd, cmd_fail)
> 
>  TEST_F(iommufd, cmd_length)
>  {
> -#define TEST_LENGTH(_struct, _ioctl)                                     \
> +#define TEST_LENGTH(_struct, _ioctl, _last)                              \
>         {                                                                \
> +               size_t min_size = offsetofend(struct _struct, _last);    \
>                 struct {                                                 \
>                         struct _struct cmd;                              \
>                         uint8_t extra;                                   \
> -               } cmd = { .cmd = { .size = sizeof(struct _struct) - 1 }, \
> +               } cmd = { .cmd = { .size = min_size - 1 },               \
>                           .extra = UINT8_MAX };                          \
>                 int old_errno;                                           \
>                 int rc;                                                  \
> --------------------------------------------------------------------------
> 
> Any misaligned size within the range of [min_size, struct_size) still
> doesn't have a coverage though. Is this something that we have to let
> it fail with a potential risk?

It looks about right, I didn't try to test all the permutations, it
could be done but I'm not sure it has value.

Jason