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

[Nicolin Chen]

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?

Thanks
Nic