Re: [Xen-devel] [PATCH v3 5/9] x86: Temporary disable SMAP to legally access user pages in kernel mode

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Monday, May 05, 2014 3:37 PM
> To: Wu, Feng
> Cc: andrew.cooper3@xxxxxxxxxx; ian.campbell@xxxxxxxxxx; Dong, Eddie;
> Nakajima, Jun; Tian, Kevin; xen-devel@xxxxxxxxxxxxx
> Subject: RE: [PATCH v3 5/9] x86: Temporary disable SMAP to legally access user
> pages in kernel mode
> 
> >>> On 05.05.14 at 09:25, <feng.wu@xxxxxxxxx> wrote:
> 
> >
> >> -----Original Message-----
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: Monday, May 05, 2014 2:57 PM
> >> To: Wu, Feng
> >> Cc: andrew.cooper3@xxxxxxxxxx; ian.campbell@xxxxxxxxxx; Dong, Eddie;
> >> Nakajima, Jun; Tian, Kevin; xen-devel@xxxxxxxxxxxxx
> >> Subject: RE: [PATCH v3 5/9] x86: Temporary disable SMAP to legally access
> > user
> >> pages in kernel mode
> >>
> >> >>> On 05.05.14 at 07:12, <feng.wu@xxxxxxxxx> wrote:
> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> >> >>> On 28.04.14 at 05:16, <feng.wu@xxxxxxxxx> wrote:
> >> >> > --- a/xen/arch/x86/x86_64/compat/entry.S
> >> >> > +++ b/xen/arch/x86/x86_64/compat/entry.S
> >> >> > @@ -266,6 +266,7 @@ ENTRY(compat_int80_direct_trap)
> >> >> >  /* On return only %rbx and %rdx are guaranteed non-clobbered.
> >> >> */
> >> >> >  compat_create_bounce_frame:
> >> >> >          ASSERT_INTERRUPTS_ENABLED
> >> >> > +        ASM_STAC
> >> >> >          mov   %fs,%edi
> >> >> >          testb $2,UREGS_cs+8(%rsp)
> >> >> >          jz    1f
> >> >>
> >> >> I think this should be deferred as much as possible; I even think it is
> >> >> warranted to put this at two places here (in the two conditional
> >> >> execution branches) just to avoid doing this too early.
> >> >
> >> > I think about this again. Seems ASM_STAC/ASM_CLAC is not needed for
> >> > compat_create_bounce_frame, since in this chunk of code, it only
> accesses
> >> > the pv guest's kernel stack, which should be in ring 1 for 32-bit pv. Is 
> >> > my
> >> > understanding correct? Thanks a lot!
> >>
> >> This is indeed correct, but at the same time means that we need to
> >> be aware that SMAP doesn't shield us at all from 32-bit PV guests, i.e.
> >> is only of limited use.
> >
> > I am not quite understand about this, why eliminating ASM_STAC/ASM_CAL
> here
> > results in " SMAP doesn't shield us at all from 32-bit PV guests "? Thanks a
> > lot!
> 
> I didn't say that; what I said was that there is no shielding (i.e.
> irrespective of the absence/presence of CLAC in this code path).

Okay, I got it.

So I think I can remove the STAC/CLAC here in the next post, right?

> 
> Jan

Thanks,
Feng

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