Re: [Xen-devel] [PATCH] x86/vm_event: correctly gather gs_shadow value

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Wed, May 1, 2019 at 7:45 AM Tamas K Lengyel <tamas@xxxxxxxxxxxxx> wrote:
>
> On Wed, May 1, 2019 at 1:50 AM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
> >
> > On 01/05/2019 05:22, Tamas K Lengyel wrote:
> > > Currently the gs_shadow value is only cached when the vCPU is being 
> > > scheduled
> > > out by Xen. Reporting this (usually) stale value through vm_event is 
> > > incorrect,
> > > since it doesn't represent the actual state of the vCPU at the time the 
> > > event
> > > was recorded. This prevents vm_event subscribers from correctly finding 
> > > kernel
> > > structures in the guest when it is trapped while in ring3.
> > >
> > > Signed-off-by: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
> > > Cc: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
> > > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > > Cc: Roger Pau Monne <roger.pau@xxxxxxxxxx>
> > > ---
> > >  xen/arch/x86/vm_event.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
> > > index 51c3493b1d..4464940da7 100644
> > > --- a/xen/arch/x86/vm_event.c
> > > +++ b/xen/arch/x86/vm_event.c
> >
> > Actually, come to think of it, the same is true for the SYSENTER
> > details, which by default are read/write to the guest while it is
> > scheduled.  As a result, the details reported here will from the last
> > vcpu context switch, and possibly stale.
>
> I'll look into it.

The sysenter values look fine to me because vmx_save_vmcs_ctxt calls
vmx_vmcs_save, which refreshes the values from the actual VMCS. It's
only shadow_gs that is not refreshed.

Tamas

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