Re: [Xen-devel] [PATCH] x86/stackframe/32: repair 32-bit Xen PV

[Andy Lutomirski]

On Thu, Oct 24, 2019 at 9:32 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 24/10/2019 17:11, Andy Lutomirski wrote:
> >> +# define USER_SEGMENT_RPL_MASK (SEGMENT_RPL_MASK & ~1)
> >> +#endif
> >> +
> >>         .section .entry.text, "ax"
> >>
> >>  /*
> >> @@ -172,7 +183,7 @@
> >>         ALTERNATIVE "jmp .Lend_\@", "", X86_FEATURE_PTI
> >>         .if \no_user_check == 0
> >>         /* coming from usermode? */
> >> -       testl   $SEGMENT_RPL_MASK, PT_CS(%esp)
> >> +       testl   $USER_SEGMENT_RPL_MASK, PT_CS(%esp)
> > Shouldn't PT_CS(%esp) be 0 if we came from the kernel?  I'm guessing
> > the actual bug is in whatever code put 1 in here in the first place.
>
> Ring1 kernels (32bit) consistently see RPL1 everywhere under Xen.
>
> Back in the days of a 32bit Xen, int $0x80 really was wired directly
> from ring 3 to 1, and didn't bounce through Xen.  This isn't possible in
> long mode, because all IDT gates are required to be 64bit code segments.
>
> Ring3 kernels (64bit) consistently see RPL0 everywhere under Xen,
> because presumably this was less invasive when designing the ABI.
>

OK, gotcha.

So I'm fine with this patch if you improve the comment and definition.