Re: [Xen-devel] [PATCH v4 08/10] x86: Enable Supervisor Mode Access Prevention (SMAP) for Xen

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

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Tuesday, May 06, 2014 4:02 PM
> To: Wu, Feng
> Cc: andrew.cooper3@xxxxxxxxxx; ian.campbell@xxxxxxxxxx; Dong, Eddie;
> Nakajima, Jun; Tian, Kevin; xen-devel@xxxxxxxxxxxxx
> Subject: RE: [PATCH v4 08/10] x86: Enable Supervisor Mode Access Prevention
> (SMAP) for Xen
> 
> >>> On 06.05.14 at 07:19, <feng.wu@xxxxxxxxx> wrote:
> 
> >
> >> -----Original Message-----
> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> >> Sent: Monday, May 05, 2014 7:20 PM
> >> To: Wu, Feng
> >> Cc: andrew.cooper3@xxxxxxxxxx; ian.campbell@xxxxxxxxxx; Dong, Eddie;
> >> Nakajima, Jun; Tian, Kevin; xen-devel@xxxxxxxxxxxxx
> >> Subject: Re: [PATCH v4 08/10] x86: Enable Supervisor Mode Access
> Prevention
> >> (SMAP) for Xen
> >>
> >> >>> On 05.05.14 at 10:23, <feng.wu@xxxxxxxxx> wrote:
> >> > @@ -1394,6 +1401,13 @@ void __init noreturn __start_xen(unsigned
> long
> >> mbi_p)
> >> >                          bootstrap_map, cmdline) != 0)
> >> >          panic("Could not set up DOM0 guest OS");
> >> >
> >> > +    /*
> >> > +     * Enable SMAP after constructing domain0, since there are lots of
> >> accesses to
> >> > +     * user pages in construct_dom0(), which is safe at the current
> stage.
> >> > +     */
> >> > +    if ( cpu_has_smap )
> >> > +        set_in_cr4(X86_CR4_SMAP);
> >> > +
> >>
> >> I should have noticed this on the previous round already - the way
> >> it's being done right now will leave APs with SMAP disabled for an
> >> indeterminate amount of time (until they first reload CR4 from
> >> mmu_cr4_features).
> >
> > Yes, that's a question. In that case, we should move this part to the place
> > where
> > it was. So we should come back to the question about how to handle
> > stac()/clac()
> > in construct_dom0(), right?
> 
> An option might be to move it back to where it was, but clear the flag
> temporarily on the one CPU calling construct_dom0(). That's a little
> hackish, but properly commented acceptable imo.

Do you mean clear SMAP bit in CR4 before construct_dom0() and set it after that 
like this?

write_cr4(read_cr4() & ~X86_CR4_SMAP);
construct_dom0();
write_cr4(read_cr4() | X86_CR4_SMAP);

> 
> Jan

Thanks,
Feng

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