Re: [kernel-hardening] [PATCH] lib: harden strncpy_from_user

[Mark Rutland <mark.rutland@arm.com>]

On Fri, Aug 26, 2016 at 02:57:58PM -0400, Kees Cook wrote:
> On Fri, Aug 26, 2016 at 10:31 AM, Mark Rutland <mark.rutland@arm.com> wrote:
> > The strncpy_from_user() accessor is effectively a copy_from_user()
> > specialised to copy strings, terminating early at a NUL byte if
> > possible. In other respects it is identical, and can be used to copy an
> > arbitrarily large buffer from userspace into the kernel. Conceptually,
> > it exposes a similar attack surface.
> >
> > As with copy_from_user(), we check the destination range when the kernel
> > is built with KASAN, but unlike copy_from_user() we do not check the
> > destination buffer when using HARDENED_USERCOPY. As strncpy_from_user()
> > calls get_user() in a loop, we must call check_object_size() explicitly.
> >
> > This patch adds this instrumentation to strncpy_from_user(), per the
> > same rationale as with the regular copy_from_user(). In the absence of
> > hardened usercopy this will have no impact as the instrumentation
> > expands to an empty static inline function.

[...]

> Ah, yes, good catch! (And to repeat what you mentioned to me in
> passing in the hall: there appear to be other users of get_user() in a
> loop in other places in the kernel that will likely need some
> attention too.)

I was reminded of this as it just hit mainline; is it worth dropping a
TODO on the KSPP wiki? I suspect I won't have the time to delve much
further into this in the near term, and it might be a good intro task
for someone.

Thanks,
Mark.