Re: [Xen-devel] [PATCH v2] xen/sched: don't let XEN_RUNSTATE_UPDATE leak into vcpu_runstate_get()

[Julien Grall <julien.grall@xxxxxxx>]

Hi,

On 9/26/19 3:45 PM, Jürgen Groß wrote:
> On 26.09.19 16:27, Julien Grall wrote:
> > Hi,
> >
> > On 9/25/19 5:29 AM, Juergen Gross wrote:
> > > vcpu_runstate_get() should never return a state entry time with
> > > XEN_RUNSTATE_UPDATE set. To avoid this let update_runstate_area()
> > > operate on a local runstate copy.
> > >
> > > This problem was introduced with commit 2529c850ea48f036 ("add update
> > > indicator to vcpu_runstate_info").
> > >
> > > Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > ---
> > > V2: add handling on ARM, too (Jan Beulich)
> > > ---
> > >   xen/arch/arm/domain.c | 13 ++++++++-----
> > >   xen/arch/x86/domain.c | 17 ++++++++++-------
> > >   2 files changed, 18 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
> > > index ae13e47e86..d681ff5c6e 100644
> > > --- a/xen/arch/arm/domain.c
> > > +++ b/xen/arch/arm/domain.c
> > > @@ -280,28 +280,31 @@ static void ctxt_switch_to(struct vcpu *n)
> > >   static void update_runstate_area(struct vcpu *v)
> > >   {
> > >       void __user *guest_handle = NULL;
> > > +    struct vcpu_runstate_info runstate;
> > >       if ( guest_handle_is_null(runstate_guest(v)) )
> > >           return;
> > > +    memcpy(&runstate, &v->runstate, sizeof(runstate));
> > I am not really happy with this solution. AFAICT, you only copy the 
> > full structure here just for the benefits of updating state_entry_time.
> > I saw you discuss about it with Jan, so it would be nice to log at 
> > least in the commit message the reason why this is done like that.
> Isn't the reference to commit 2529c850ea48f036 enough? The update
> protocol is clearly described in that commit message.
I meant the reason to use the 'memcpy', which sounds like quite a waste, 
compare to only copy state_entry_time.
Cheers,

--
Julien Grall

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