Re: [PATCH] x86/entry_32: Move CLEAR_CPU_BUFFERS before CR3 switch

[Pawan Gupta]

On Thu, May 09, 2024 at 05:20:31PM -0700, Dave Hansen wrote:
> On 4/26/24 16:48, Pawan Gupta wrote:
> > Move the VERW before the CR3 switch for 32-bit kernels as a workaround.
> 
> I look at the 32-bit code so rarely, I seem to forget have to re-learn
> this gunk every time I look at it.  Take a look at RESTORE_INT_REGS.  On
> 32-bit, we actually restore %ds:
> 
> 	popl    %ds
> 
> So even doing this:
> 
> > +       CLEAR_CPU_BUFFERS
> >         /* Restore user state */
> >         RESTORE_REGS pop=4                      # skip orig_eax/error_code
> > -       CLEAR_CPU_BUFFERS
> >  .Lirq_return:
> 
> fixes the issue.  Moving it above the CR3 switch also works of course,
> but I don't think this has anything to do with CR3.  It's just that
> userspace sets a funky %ds value and CLEAR_CPU_BUFFERS uses ds:.

I will test it out, but I think you are right. VERW documentation says:

  #GP(0)	If a memory operand effective address is outside the CS,
		DS, ES, FS, or GS segment limit.

> I don't think any of the segment registers can have secrets in them, can
> they?  I mean, it's possible, but in practice I can't imagine.

I don't think so they are secrets. AFAICT, their values are build-time
constants, and can be easily deduced.

> So why not just do the CLEAR_CPU_BUFFERS in RESTORE_REGS but after
> RESTORE_INT_REGS?  You might be able to do it universally, or you could
> pass in a macro argument to do it conditionally.

Sounds good. I will try that, possibly tomorrow.

> P.S. Can we remove 32-bit support yet?  Please? :)

+1 ... or atleast the mitigations for 32-bit :)