Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

[Joerg Roedel]

On Mon, Mar 05, 2018 at 04:17:45AM -0800, Linus Torvalds wrote:
>     Restoring the segments can cause exceptions that need to be
>     handled. With PTI enabled, we still need to be on kernel cr3
>     when the exception happens. For the cr3-switch we need
>     at least one integer scratch register, so we can't switch
>     with the user integer registers already loaded.
> 
> 
> This fundamentally seems wrong.

Okay, right, with v3 it is wrong, in v2 I still thought I could get away
without remembering the entry-cr3, but didn't think about the #DB case
then.

In v3 I added code which remembers the entry-cr3 and handles the
entry-from-kernel-mode-with-user-cr3 case for all exceptions including
#DB.

> The things is, we *know* that we will restore two segment registers with the
> user cr3 already loaded: CS and SS get restored with the final iret.

Yeah, I know, but the iret-exception path is fine because it will
deliver a SIGILL and doesn't return to the faulting iret.

Anyway, I will remove these restore-reorderings, they are not needed
anymore.

> So has this been tested with
> 
>  - single-stepping through sysenter
> 
>    This takes a DB fault in the first kernel instruction. We're in kernel mode,
> but with user cr3.
> 
>  - ptracing and setting CS/SS to something bad
> 
>    That should test the "exception on iret" case - again in kernel mode, but
> with user cr3 restored for the return.

The iret-exception case is tested by the ldt_gdt selftest (the
do_multicpu_tests subtest). But I didn't actually tested single-stepping
through sysenter yet. I just re-ran the same tests I did with v2 on this
patch-set.

Regards,

	Joerg