Re: [Xen-devel] [PATCH v9 06/11] x86/entry: Organise the use of MSR_SPEC_CTRL at each entry/exit point

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 01/22/2018 07:17 PM, Andrew Cooper wrote:
> On 22/01/2018 22:27, Boris Ostrovsky wrote:
> > On 01/19/2018 08:36 AM, Andrew Cooper wrote:
> > > On 19/01/18 11:43, Jan Beulich wrote:
> > >
> > > > > @@ -99,6 +106,10 @@ UNLIKELY_END(realmode)
> > > > >   .Lvmx_vmentry_fail:
> > > > >           sti
> > > > >           SAVE_ALL
> > > > > +
> > > > > +        SPEC_CTRL_ENTRY_FROM_PV /* Req: %rsp=regs/cpuinfo Clob: acd */
> > > > I think the use of the PV variant here requires a comment.
> > > Oh.  It used to have one...  I'll try to find it.
> > I, in fact, meant to ask about this for a long time and always forgot.
> > Perhaps your comment will say more than just why a PV variant is used
> > here but in case it won't --- why do we have *any* mitigation here? We
> > are never returning to the guest, do we?
> We never return to *this* guest, but we are still open to abuse from a
> separate hyperthread, so still need to set SPEC_CTRL.IBRS if we are
> using IBRS for safety.  (If we are using lfence+jmp or repoline then we
> don't need this change, but its not a hotpath so doesn't warrant yet
> another variant of SPEC_CTRL_ENTRY_FROM_*.)

We wrote IBRS during VMEXIT. I thought this serves as barrier for all 
preceding predictions (both threads) from lower protection mode.
Is the concern here that SPEC_CTRL_EXIT_TO_GUEST (before VMENTER) may 
set IBRS to 0 and *that* will open hypervisor to other thread's mischief?
-boris

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