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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel