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

From: Alexandre Chartre
Date: Mon Nov 09 2020 - 14:54:17 EST



[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.