Re: [Xen-devel] [PATCH v2 3/7] 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: Wednesday, April 23, 2014 10:13 PM
> To: Andrew Cooper; Wu, Feng
> Cc: ian.campbell@xxxxxxxxxx; Dong, Eddie; Nakajima, Jun; Tian, Kevin;
> xen-devel@xxxxxxxxxxxxx
> Subject: RE: [PATCH v2 3/7] x86: Temporary disable SMAP to legally access user
> pages in kernel mode
> 
> >>> On 23.04.14 at 15:43, <feng.wu@xxxxxxxxx> wrote:
> >> From: Andrew Cooper [mailto:andrew.cooper3@xxxxxxxxxx]
> >> On 23/04/14 15:35, Feng Wu wrote:
> >> > --- a/xen/arch/x86/domain_build.c
> >> > +++ b/xen/arch/x86/domain_build.c
> >> > @@ -778,6 +778,7 @@ int __init construct_dom0(
> >> >      }
> >> >      bootstrap_map(NULL);
> >> >
> >> > +    stac();
> >>
> >> As constructing dom0 is trusted, this should be near the top of top of
> >> the function
> >
> > We cannot call stac() near the top of the function, because construct_dom0()
> > calls
> > elf_load_binary() which calls copy_from_user(), we can only add stac() after
> > calling
> > elf_load_binary(), otherwise the AC bit will remain cleared after
> > elf_load_binary().
> >
> > I just sugguest another method in another mail, can you please have a look?
> 
> But that other method widened the scope even further, so would suffer
> the same issue. How about enabling SMAP only after having built Dom0?

Yes, it suffers the same problem. Enabling SMAP after constructing dom0 may
be a good suggestion.
> 
> Jan

Thanks,
Feng


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