Re: [kernel-hardening] Re: [RFC v1] implement SL*B and stack usercopy runtime checks

Vasiliy Kulikov
2011-07-03T19:24:42Z

On Sun, Jul 03, 2011 at 12:10 -0700, Linus Torvalds wrote:
> On Sun, Jul 3, 2011 at 11:57 AM, Vasiliy Kulikov <segoon@openwall.com> wrote:
> >> If you seriously clean it up (that at a minimum includes things like
> >> making it configurable using some pretty helper function that just
> >> compiles away for all the normal cases,
> >
> > Hm, it is not as simple as it looks at the first glance - even if the
> > object size is known at the compile time (__compiletime_object_size), it
> > might be a field of a structure, which crosses the slab object
> > boundaries because of an overflow.
>
> No, I was more talking about having something like
>
> #ifdef CONFIG_EXPENSIVE_CHECK_USERCOPY
> extern int check_user_copy(const void *kptr, unsigned long size);
> #else
> static inline int check_user_copy(const void *kptr, unsigned long size)
> { return 0; }
> #endif

Sure, will do. This is what I mean by kernel_access_ok() as it is a
weak equivalent of access_ok(), check_user_copy() is a bit confusing
name IMO.

> so that the actual user-copy routines end up being clean and not have
> #ifdefs in them or any implementation details like what you check
> (stack, slab, page cache - whatever)
>
> If you can also make it automatically not generate any code for cases
> that are somehow obviously safe, then that's an added bonus.

OK, then let's stop on "checks for overflows" and remove the check if
__compiletime_object_size() says something or length is constant. It
should remove most of the checks in fast pathes.

> But my concern is that performance is a real issue, and the strict
> user-copy checking sounds like mostly a "let's enable this for testing
> kernels when chasing some particular issue" feature, the way
> DEBUG_PAGEALLOC is.

I will measure the perfomance penalty tomorrow.

Btw, if the perfomance will be acceptable, what do you think about
logging/reacting on the spotted overflows?

Thanks,

--
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments