Re: [PATCH 2/4] KVM: x86: Introduce paravirt feature CR0/CR4 pinning

[Kees Cook]

On Thu, Jul 09, 2020 at 09:22:09AM -0700, Dave Hansen wrote:
> On 7/9/20 9:07 AM, Andy Lutomirski wrote:
> > On Thu, Jul 9, 2020 at 8:56 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> >> On 7/9/20 8:44 AM, Andersen, John wrote:
> >>>         Bits which are allowed to be pinned default to WP for CR0 and SMEP,
> >>>         SMAP, and UMIP for CR4.
> >> I think it also makes sense to have FSGSBASE in this set.
> >>
> >> I know it hasn't been tested, but I think we should do the legwork to
> >> test it.  If not in this set, can we agree that it's a logical next step?
> > I have no objection to pinning FSGSBASE, but is there a clear
> > description of the threat model that this whole series is meant to
> > address?  The idea is to provide a degree of protection against an
> > attacker who is able to convince a guest kernel to write something
> > inappropriate to CR4, right?  How realistic is this?
> 
> If a quick search can find this:
> 
> > https://googleprojectzero.blogspot.com/2017/05/exploiting-linux-kernel-via-packet.html
> 
> I'd pretty confident that the guys doing actual bad things have it in
> their toolbox too.

Right, it's common (see my commit log in 873d50d58f67), and having this
enforced by the hypervisor is WAY better since it'll block gadgets or
ROP.

-- 
Kees Cook