Re: [Xen-devel] [PATCH] xen/pvshim: fix GNTTABOP_query_size hypercall forwarding with SMAP

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Jan 26, 2018 at 03:52:38PM +0000, Wei Liu wrote:
> On Fri, Jan 26, 2018 at 03:29:10PM +0000, Roger Pau Monne wrote:
> > Disable SMAP in the shim before bouncing the hypercall, or else L0
> > will fail to get the hypercall buffer.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > Reported-by: Fatih Acar <fatih.acar@xxxxxxxxx>
> > ---
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > ---
> > Needs to be backported to the 4.10.0-shim-comet branch.
> > ---
> >  xen/arch/x86/pv/shim.c | 3 +++
> >  1 file changed, 3 insertions(+)
> > 
> > diff --git a/xen/arch/x86/pv/shim.c b/xen/arch/x86/pv/shim.c
> > index d5383dcfc7..eb1ee7d3c4 100644
> > --- a/xen/arch/x86/pv/shim.c
> > +++ b/xen/arch/x86/pv/shim.c
> > @@ -748,7 +748,10 @@ static long pv_shim_grant_table_op(unsigned int cmd,
> >      }
> >  
> >      case GNTTABOP_query_size:
> > +        /* Disable SMAP so L0 can access the buffer. */
> 
> Interesting.
> 
> > +        stac();
> >          rc = xen_hypercall_grant_table_op(GNTTABOP_query_size, uop.p, 
> > count);
> > +        clac();
> 
> There is so far only one instance of this, so doing this ad-hoc stac /
> clac is OK.
> 
> Probably another (more uniformed) way of fixing it is to have our own
> buffer (forbid passing through uop directly) and the copy back the
> result. Or we should invent a set of macros to deal with uop.

I guess we should figure out what's faster, disabling SMAP or copying
to a buffer?

Thanks, Roger.

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