Re: [RFC] x86: Avoid CR3 load on compatibility mode with PTI

[Nadav Amit]

Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> 
>> On Jan 14, 2018, at 12:13 PM, Nadav Amit <namit@xxxxxxxxxx> wrote:
>> 
>> Currently, when page-table isolation is on to prevent the Meltdown bug
>> (CVE-2017-5754), CR3 is always loaded on system-call and interrupt.
>> 
>> However, it appears that this is an unnecessary measure when programs
>> run in compatibility mode. In this mode only 32-bit registers are
>> available, which means that there *should* be no way for the CPU to
>> access, even speculatively, memory that belongs to the kernel, which
>> sits in high addresses.
> 
> You're assuming that TIF_IA32 prevents the execution of 64-bit code.  It doesn't.
> 
> I've occasionally considered adding an opt-in hardening mechanism to enforce 32-bit or 64-bit execution, but we don't have this now.

I noticed it doesnât. I thought the removing/restoring the __USER_CS
descriptor on context switch, based on TIF_IA32, would be enough.
modify_ldt() always keeps the descriptor l-bit clear. I will review the
other GDT descriptors, and if needed, create two GDTs. Let me know if I
missed anything else.

> Anything like this would also need to spend on SMEP, I think -- the pseudo-SMEP granted by PTI is too valuable to give up on old boxes, I think.

If SMEP is not supported, compatibility mode would still require page-table
isolation.

Thanks for the feedback. I still look for an ack for the basic idea of
disabling page-table isolation on compatibility mode.

Regards,
Nadav