Re: [PATCH RFC v2 4/6] x86: Disable PTI on compatibility mode

[Andrew Cooper]

On 16/02/2018 00:08, Linus Torvalds wrote:
> On Thu, Feb 15, 2018 at 3:29 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>> Linus, how would you feel about, by default, preventing 64-bit
>> programs from long-jumping to __USER32_CS and vice versa?
> How? It's a standard GDT entry. Are you going to start switching the
> GDT around every context switch?
>
> I *thought* that user space can just do a far jump on its own. But
> it's so long since I had to care that I may have forgotten all the
> requirements for going between "compatibility mode" and real long
> mode.

Yes - it is just a straight far jump to switch between compat and long mode.

A evil^W cunning programmer can use the 286 world view and disable
segments by clearing the present bit to yield #NP[sel] on use, which is
liable to be rather faster than LGDT on a context switch.

Alternatively, set both the L and D (code segments only), or playing
with DPL/type can all yield #GP[sel] on use, but these probably aren't
as good options.

~Andrew