Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on long-mode

[Nadav Amit]

On 6/16/14, 5:56 PM, Paolo Bonzini wrote:
> Il 16/06/2014 13:53, Nadav Amit ha scritto:
> > On 6/16/14, 2:09 PM, Paolo Bonzini wrote:
> > > Il 16/06/2014 12:33, Nadav Amit ha scritto:
> > > > > Do you get this if the input register has bit 31 set?
> > > > No. To be frank, the scenario may be considered a bit synthetic: the
> > > > guest assigns a value to a general-purpose register in 64-bit mode,
> > > > setting the high 32-bits to some non-zero value. Then, later, in 32-bit
> > > > mode, the guest performs MOV DR instruction. In between the two
> > > > assignments, the general purpose register is unmodified, so the high
> > > > 32-bits of the general purpose registers are still set.
> > > >
> > > > Note that this scenario does not occur when MOV DR is emulated, but
> > > > when
> > > > handle_dr() is called. In this case, the entire 64-bits of the general
> > > > purpose register used for MOV DR are read, regardless to the execution
> > > > mode of the guest.
> > >
> > > I wonder if the same bug happens elsewhere.  For example,
> > > kvm_emulate_hypercall doesn't look at CS.L/CS.DB, which is really a
> > > corner case but arguably also a bug.  kvm_hv_hypercall instead does it
> > > right.
> > >
> > > Perhaps we need a variant of kvm_register_read that (on 64-bit hosts)
> > > checks EFER/CS.L/CS.DB and masks the returned value accordingly.  You
> > > could call it kvm_register_readl.
> >
> > There are two questions that come in mind:
> > 1. Should we ignore CS.DB? It would make it consistent with
> > kvm_hv_hypercall and handle_dr. I think this is the proper behavior.
>
> It depends on what you're using it for, but as a start yes.
>
> > 2. Reading CS.L once and masking all the registers (i.e., changing the
> > is_long_mode in kvm_emulate_hypercall to is_64_bit_mode) is likely to be
> > more efficient.
>
> Yes, for the case of kvm_emulate_hypercall.  Then you can build
> kvm_register_readl on top of is_64bit_mode and fix this bug with that
> function.  Did you check that handle_cr is unaffected?

handle_cr is affected, and there are some other instances of affected 
register reads. Actually, most of the vmx instruction exit handling 
routines ignore long-mode and cs.l when considering the register operand 
size.

I'll make the necessary changes and resubmit.

Nadav

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/