Re: [Xen-devel] [PATCH 2/2] x86/hvm: honor guest's option when updating secondary system time for guest

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

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Friday, July 25, 2014 3:26 PM
> To: Wu, Feng
> Cc: linux@xxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx;
> keir@xxxxxxx; tim@xxxxxxx
> Subject: RE: [PATCH 2/2] x86/hvm: honor guest's option when updating
> secondary system time for guest
> 
> >>> On 25.07.14 at 06:30, <feng.wu@xxxxxxxxx> wrote:
> 
> >
> >> -----Original Message-----
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: Wednesday, July 23, 2014 8:19 PM
> >> To: Wu, Feng
> >> Cc: linux@xxxxxxxxxxxxxx; xen-devel@xxxxxxxxxxxxx;
> konrad.wilk@xxxxxxxxxx;
> >> keir@xxxxxxx; tim@xxxxxxx
> >> Subject: Re: [PATCH 2/2] x86/hvm: honor guest's option when updating
> >> secondary system time for guest
> >>
> >> >>> On 08.07.14 at 01:18, <feng.wu@xxxxxxxxx> wrote:
> >> > --- a/xen/include/public/vcpu.h
> >> > +++ b/xen/include/public/vcpu.h
> >> > @@ -227,6 +227,16 @@ struct vcpu_register_time_memory_area {
> >> >  typedef struct vcpu_register_time_memory_area
> >> > vcpu_register_time_memory_area_t;
> >> >  DEFINE_XEN_GUEST_HANDLE(vcpu_register_time_memory_area_t);
> >> >
> >> > +/*
> >> > + * Flags to tell Xen whether we need to do the SMAP check when
> updating
> >> > + * the secondary copy of the vcpu time when SMAP is enabled. Since the
> >> > + * memory location for the secondary copy of the vcpu time may be
> mapped
> >> > + * into userspace by guests intendedly, we let the guest to determine
> >> > + * whether the check is needed. The default behavior of hypevisor is
> >> > + * not doing the check.
> >> > + */
> >> > +#define VCPUOP_enable_smap_check_vcpu_time_memory_area   14
> >>
> >> I think the new op to be VCPUOP_register_vcpu_time_memory_area_smap,
> >> identical to VCPUOP_register_vcpu_time_memory_area apart from also
> >> setting the flag, would be more natural. But considering what I just wrote
> >> in the reply to Tim I guess we can expect a nun-user mapping to be
> >> presented here anyway, i.e. we wouldn't need to new operation at all.
> >
> > Do you mean since the user-paging is r/o, guest will pass a r/w kernel page
> > to
> > Xen for updating the system time. So we don't need to do the SMAP check
> > in this case?
> 
> If the user page is r/o, it's VA obviously can't be used for updating by
> Xen. Hence the kernel has to provide a r/w mapped VA. That should be
> subject to SMAP checking (consistent with the runstate area handling),
> to make sure it's not a user accessible mapping.

But there are two possible problems here:
1. Is it possible that guest passes a user r/w page to update the system time 
information?
2. Even the user page is r/o, the kernel can still use it to update the system 
time info when WP is disabled.

Thanks,
Feng

> 
> Jan


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