Re: [PATCH v10 1/3] x86/tlb: introduce a flush HVM ASIDs flag

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

On Thu, Apr 23, 2020 at 12:41:49PM +0200, Jan Beulich wrote:
> On 23.04.2020 12:30, Wei Liu wrote:
> > On Wed, Apr 22, 2020 at 06:33:38PM +0200, Roger Pau Monné wrote:
> >> On Thu, Apr 16, 2020 at 03:59:07PM +0200, Roger Pau Monne wrote:
> >>> @@ -254,3 +257,14 @@ unsigned int flush_area_local(const void *va, 
> >>> unsigned int flags)
> >>>  
> >>>      return flags;
> >>>  }
> >>> +
> >>> +void guest_flush_tlb_mask(const struct domain *d, const cpumask_t *mask)
> >>> +{
> >>> +    unsigned int flags = (is_pv_domain(d) || paging_mode_shadow(d) ? 
> >>> FLUSH_TLB
> >>> +                                                                   : 0) |
> >>> +                         (is_hvm_domain(d) && cpu_has_svm ? 
> >>> FLUSH_HVM_ASID_CORE
> >>> +                                                          : 0);
> >>
> >> Maybe I'm getting confused, but I think the above is wrong and ASID
> >> should _always_ be flushed when running a HVM domain in shadow mode
> >> regardless of whether the underlying hw is Intel or AMD, ie:
> >>
> >> bool shadow = paging_mode_shadow(d);
> >> unsigned int flags = (shadow ? FLUSH_TLB : 0) |
> >>                      (is_hvm_domain(d) &&
> >>                       (cpu_has_svm || shadow) ? FLUSH_HVM_ASID_CORE : 0);
> > 
> > This sort of long expression is prone to error. See XSA-316.
> 
> To be honest I consider it quite fine. XSA-316 was in particular
> because of successive closing parentheses, of which there are
> none here. (This isn't to say I would strictly mind splitting,
> but I fear this would result in (multiple?) single use local
> variables.)

Right now it's exactly (including the indentation):

    bool shadow = paging_mode_shadow(d);

    return (shadow ? FLUSH_TLB : 0) |
           (is_hvm_domain(d) && (cpu_has_svm || shadow) ? FLUSH_HVM_ASID_CORE
                                                        : 0);

I could change it to:

    bool shadow = paging_mode_shadow(d);
    bool asid = is_hvm_domain(d) && (cpu_has_svm || shadow);

    return (shadow ? FLUSH_TLB : 0) | (asid ? FLUSH_HVM_ASID_CORE : 0);

But would result in a single use asid local variable.

I think XSA-316 was slightly different because the issue arose from
assigning and comparing a variable inside of an if condition, which is
not the case here. I however don't mind changing it if it's regarded
as potentially insecure, or hard to read.

Thanks, Roger.