Re: [PATCHv2 1/1] Documentation: describe how to add a system call

[Josh Triplett <josh@joshtriplett.org>]

On Thu, Jul 30, 2015 at 01:03:43PM -0700, Kees Cook wrote:
> On Thu, Jul 30, 2015 at 12:04 PM, Josh Triplett <josh@joshtriplett.org> wrote:
> > On Thu, Jul 30, 2015 at 11:21:54AM -0700, Kees Cook wrote:
> >> I like this, it's a good description of both options. I'm still biased
> >> about the approach: I prefer flags, since pointers to user structures
> >> complicate syscall filtering. ;)
> >
> > Seems like we should do two things to make that easier:
> >
> > 1) Create a standardized kernel mechanism for parameter-struct handling,
> >    implementing the recommendations mentioned here.
> 
> It's been suggested in the past that nlmsg is appropriate for such a
> thing, but I remain suspicious. :)

Likewise. :)

> > 2) Integrate into that mechanism a way to filter the resulting parameter
> >    struct with BPF *after* it has been copied to kernel space (and thus
> >    can no longer be tampered with).
> 
> Yeah, this is a irritating part: the structures operated on are copied
> from userspace adhoc in each syscall. Doing argument checking would
> mean double copies initially, and perhaps teaching syscalls about
> optional "already copied" arguments or something as an optimization.

No, double copies can't work for security reasons.  Because otherwise
you could race the kernel from another thread, substituting different
values after the check and before the use.

I think the right API looks *roughly* like this:

int _copy_param_struct(size_t kernel_len, void *kernel_struct, size_t user_len, void __user *user_struct)
{
	if (user_len > kernel_len)
		return -EINVAL;
	if (user_len && copy_from_user(kernel_struct, user_struct, user_len))
		return -EFAULT;
	if (user_len < kernel_len)
		memset(kernel_struct + user_len, 0, kernel_len - user_len);
	return 0;
}

#define copy_param_struct(kernel_struct, user_len, user_struct) _copy_param_struct( \
		sizeof(*kernel_struct) + BUILD_BUG_ON_ZERO(!__same_type(*kernel_struct, *user_struct)), \
		kernel_struct, user_len, user_struct)


Then the syscall looks like this:

SYSCALL_DEFINEn(xyzzy, ..., ..., size_t user_params_len, struct xyzzy_params __user *user_params)
{
	int ret;
	struct xyzzy_params params;

	ret = copy_param_struct(&params, user_params_len, user_params);
	if (ret)
		return ret;
	...


And you could then hook copy_params_struct to add arbitrary additional
syscall parameter validation.  Bonus if there's some way to make the
copy and validation occur before the syscall is ever invoked, rather
than inside the syscall, but that would require adding fancier syscall
definition mechanisms that autogenerate such code.

- Josh Triplett