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

[Nicolin Chen]

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?

Thanks
Nic