Re: [PATCH v5] x86/dom0: disable SMAP for PV domain building only

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

On Tue, Aug 27, 2024 at 03:04:54PM +0200, Jan Beulich wrote:
> On 27.08.2024 14:59, Andrew Cooper wrote:
> > On 27/08/2024 1:39 pm, Roger Pau Monne wrote:
> >> --- a/xen/arch/x86/dom0_build.c
> >> +++ b/xen/arch/x86/dom0_build.c
> >> @@ -612,7 +612,24 @@ int __init construct_dom0(struct domain *d, const 
> >> module_t *image,
> >>      if ( is_hvm_domain(d) )
> >>          rc = dom0_construct_pvh(d, image, image_headroom, initrd, 
> >> cmdline);
> >>      else if ( is_pv_domain(d) )
> >> +    {
> >> +        /*
> >> +         * Temporarily clear SMAP in CR4 to allow user-accesses in
> >> +         * construct_dom0().  This saves a large number of corner cases
> >> +         * interactions with copy_from_user().
> >> +         */
> >> +        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
> >> +        {
> >> +            cr4_pv32_mask &= ~X86_CR4_SMAP;
> >> +            write_cr4(read_cr4() & ~X86_CR4_SMAP);
> >> +        }
> >>          rc = dom0_construct_pv(d, image, image_headroom, initrd, cmdline);
> >> +        if ( boot_cpu_has(X86_FEATURE_XEN_SMAP) )
> >> +        {
> >> +            write_cr4(read_cr4() | X86_CR4_SMAP);
> >> +            cr4_pv32_mask |= X86_CR4_SMAP;
> >> +        }
> >> +    }
> > 
> > I hate to drag this on further still, but can this logic be move it into
> > dom0_construct_pv() itself, rather than here?
> 
> Just to mention it: I'm fine with this in principle, as long as this won't
> mean a pile of new goto-s in dom0_construct_pv(). If a new wrapper was
> introduced (with the present function becoming static), I'd be okay.

I've considered adding this inside of dom0_construct_pv(), but then I
would need to adjust the return paths to re-enable SMAP.

I can add a wrapper, I didn't do it that way because it seemed
cumbersome IMO.

I will prepare v6 then with that approach.

Thanks, Roger.