Re: [RFC PATCH v2 6/6] x86/entry/pti: don't switch PGD on when pti_disable is set

[Dave Hansen]

On 01/11/2018 10:21 AM, Alexei Starovoitov wrote:
>> I was thinking that maybe we should add a new field or two to pt_regs.
>> They could store CR2 and maybe CR3 as well.  I'd also like to expose
>> the error code of exceptions in stack traces.  We should get this
>> integrated right into the unwinder.
> hmm. Exposing cr3 to user space will make it trivial for user process
> to know whether kpti is active. Not sure how exploitable such
> information leak.

It also gives userspace a pretty valuable physical address to go after.
That, plus a KASLR defeat gives you a known-valuable virtual address to
target.  That's no good.

I think CR3 is a non-starter.