Re: [PATCH] KVM: fix sil/dil/bpl/spl in the mod/rm fields

[Gleb Natapov <gleb@redhat.com>]

On Mon, Jun 03, 2013 at 08:30:18PM +0300, Avi Kivity wrote:
> On Jun 3, 2013 7:41 PM, "Gleb Natapov" <gleb@redhat.com> wrote:
> >
> > On Mon, Jun 03, 2013 at 06:42:11PM +0300, Avi Kivity wrote:
> > > On Thu, May 30, 2013 at 7:34 PM, Paolo Bonzini <pbonzini@redhat.com>
> wrote:
> > > > Il 30/05/2013 17:34, Paolo Bonzini ha scritto:
> > > >> Il 30/05/2013 16:35, Paolo Bonzini ha scritto:
> > > >>> The x86-64 extended low-byte registers were fetched correctly from
> reg,
> > > >>> but not from mod/rm.
> > > >>>
> > > >>> This fixes another bug in the boot of RHEL5.9 64-bit, but it is
> still
> > > >>> not enough.
> > > >>
> > > >> Well, it is enough but it takes 2 minutes to reach the point where
> > > >> hardware virtualization is used.  It is doing a lot of stuff in
> > > >> emulation mode because FS and GS have leftovers from the A20 test:
> > > >>
> > > >> FS =0000 0000000000000000 0000ffff 00009300 DPL=0 DS16 [-WA]
> > > >> GS =ffff 00000000000ffff0 0000ffff 00009300 DPL=0 DS16 [-WA]
> > > >>
> > > >> 0x00000000000113be:  in     $0x92,%al
> > > >> 0x00000000000113c0:  or     $0x2,%al
> > > >> 0x00000000000113c2:  out    %al,$0x92
> > > >> 0x00000000000113c4:  xor    %ax,%ax
> > > >> 0x00000000000113c6:  mov    %ax,%fs
> > > >> 0x00000000000113c8:  dec    %ax
> > > >> 0x00000000000113c9:  mov    %ax,%gs
> > > >> 0x00000000000113cb:  inc    %ax
> > > >> 0x00000000000113cc:  mov    %ax,%fs:0x200
> > > >> 0x00000000000113d0:  cmp    %gs:0x210,%ax
> > > >> 0x00000000000113d5:  je     0x113cb
> > > >>
> > > >> The DPL < RPL test fails.  Any ideas?  Should we introduce a new
> > > >> intermediate value for emulate_invalid_guest_state (0=none, 1=some,
> 2=full)?
> > > >
> > > > One idea could be to replace invalid descriptors with NULL ones.  Then
> > > > you can intercept this in the #GP handler and trigger emulation for
> that
> > > > instruction only.
> > >
> > > Won't work, vmx won't let you enter in such a configuration.
> > >
> > Why? It is possible to have NULL descriptor in 32bit mode with vmx. But
> > we do not usually intercept #GP while executing 32bit mode, so we will
> > have to track if there is artificial NULL selector and enables #GP
> > interception and then emulate on every #GP.
> 
> Sorry, was thinking of virtual-8086 mode. It should work.
> 
> >
> > > Maybe you can detect the exact code sequence (%eip, some instructions,
> > > register state) and clear %fs and %gs.
> > My be we can set dpl to rpl unconditionally on a switch from 16 to 32
> > bit. The only problem I can see with it is that if a guest enters user
> > mode without explicitly reload the segment it will be accessible by a
> > user mode code, but I am not sure it is well defined what dpl of a 16
> > bit segment is after transition to 32 bit mode anyway, so it would be
> > crazy to do so.
> 
> The problem is you cannot detect a segment reload if you do that.Trapping
> #GP preserves correctness in all cases (at the cost of some complexity).
> 
I do not see why I would want to detect reload. Setting segment to NULL
has a disadvantage that if guest will read selector it will get wrong
value, but may be we can leave selector alone and mark segment unusable.
I always wondered what VMX has "unusable" attribute for, may be this is
it.

--
			Gleb.