Re: [RFC][PATCH 13/24] x86/pti: Extend PTI user mappings

[Alexandre Chartre]

[Copying the reply to Andy in the thread with the right email addresses]

On 11/9/20 6:28 PM, Andy Lutomirski wrote:
> On Mon, Nov 9, 2020 at 3:22 AM Alexandre Chartre
> <alexandre.chartre@xxxxxxxxxx> wrote:
> > Extend PTI user mappings so that more kernel entry code can be executed
> > with the user page-table. To do so, we need to map syscall and interrupt
> > entry code,
>
> Probably fine.
>
> > per cpu offsets (__per_cpu_offset, which is used some in
> > entry code),
>
> This likely already leaks due to vulnerable CPUs leaking address space
> layout info.

I forgot to update the comment, I am not mapping __per_cpu_offset anymore.

However, if we do map __per_cpu_offset then we don't need to enforce the
ordering in paranoid_entry to switch CR3 before GS.

> > the stack canary,
>
> That's going to be a very tough sell.

I can get rid of this, but this will require to disable stack-protector for
any function that we can call while using the user page-table, like already
done in patch 21 (x86/entry: Disable stack-protector for IST entry C handlers).

alex.