[Ksummit-discuss] [CORE TOPIC] More useful types in the linux kernel

[Michael S. Tsirkin]

On Fri, Aug 12, 2016 at 03:23:28PM +1000, NeilBrown wrote:
> On Fri, Aug 12 2016, Michael S. Tsirkin wrote:
> 
> > On Tue, Jul 19, 2016 at 10:32:51AM -0500, Eric W. Biederman wrote:
> >> I would really like to get a feel among kernel maintainers and
> >> developers if this is something that is interesting, and what kind of
> >> constraints they think something like this would need to be usable for
> >> the kernel?
> >> 
> >> Eric
> >
> > Surprised that no one mentioned this yet - I think tagging
> > integers/structs as coming from userspace could be useful,
> > if we can teach e.g. smatch that access to a kernel
> > pointer through this offset might fault.
> 
> We already have that.
> Sparse recognizes
>     __attribute__((noderef, address_space(1)))
>  to mean "this is a pointer to a different address space which
>  cannot be dereferened" and linux has
> 
> # define __user                __attribute__((noderef, address_space(1)))
> 
> so if you mark a pointer as "__user", then sparse will complain
> if you dereference it.
> 
> We've had this for over a decade :-)
> 
>   https://lwn.net/Articles/87538/
> 
> NeilBrown


Of course, everyone uses these.  But what I mean is tagging index types:

	int data[256];

	int foo(u32 __user *ptr)
	{
		u32 i;
		if (get_user(i, ptr))
			return -EFAULT;

		data[i] = 0;
			^^^ security vulnerability

	}

Above, i is coming from userspace and so must always be range-checked
before it's used as an index.

Maybe we could change get_user return a tagged result: __from_user int.
And have above warn because __from_user can not be assigned to plain
int.

Then rework the code along the following lines:


	int data[256];

	int force_range(__unsafe u32 value, unsigned idx)
	{
		return ((__force int)value) % idx;
	}

	int foo(u32 __user *ptr)
	{
		__unsafe u32 i;
		int ichecked;
		if (get_user(i, ptr))
			return -EFAULT;

		ichecked = force_range(i, sizeof data);
		data[ichecked] = 0;
			^^^ ok now

	}


-- 
MST