Re: [PATCH 3/3] KVM: x86 emulator: Allow PM/VM86 switch during task switch

[Gleb Natapov <gleb@redhat.com>]

On Tue, Jan 24, 2012 at 12:31:48PM +0100, Kevin Wolf wrote:
> Am 24.01.2012 11:57, schrieb Gleb Natapov:
> > On Mon, Jan 23, 2012 at 05:10:48PM +0100, Kevin Wolf wrote:
> >> Task switches can switch between Protected Mode and VM86. The current
> >> mode must be updated during the task switch emulation so that the new
> >> segment selectors are interpreted correctly and privilege checks
> >> succeed.
> >>
> >> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> >> ---
> >>  arch/x86/include/asm/kvm_emulate.h |    1 +
> >>  arch/x86/kvm/emulate.c             |   17 +++++++++++++++++
> >>  arch/x86/kvm/x86.c                 |    6 ++++++
> >>  3 files changed, 24 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
> >> index c8a9cf3..4a21c7d 100644
> >> --- a/arch/x86/include/asm/kvm_emulate.h
> >> +++ b/arch/x86/include/asm/kvm_emulate.h
> >> @@ -176,6 +176,7 @@ struct x86_emulate_ops {
> >>  	void (*set_idt)(struct x86_emulate_ctxt *ctxt, struct desc_ptr *dt);
> >>  	ulong (*get_cr)(struct x86_emulate_ctxt *ctxt, int cr);
> >>  	int (*set_cr)(struct x86_emulate_ctxt *ctxt, int cr, ulong val);
> >> +	void (*set_rflags)(struct x86_emulate_ctxt *ctxt, ulong val);
> >>  	int (*cpl)(struct x86_emulate_ctxt *ctxt);
> >>  	int (*get_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong *dest);
> >>  	int (*set_dr)(struct x86_emulate_ctxt *ctxt, int dr, ulong value);
> >> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> >> index 833969e..52fce89 100644
> >> --- a/arch/x86/kvm/emulate.c
> >> +++ b/arch/x86/kvm/emulate.c
> >> @@ -2273,6 +2273,23 @@ static int load_state_from_tss32(struct x86_emulate_ctxt *ctxt,
> >>  		return emulate_gp(ctxt, 0);
> >>  	ctxt->_eip = tss->eip;
> >>  	ctxt->eflags = tss->eflags | 2;
> >> +
> >> +	/*
> >> +	 * If we're switching between Protected Mode and VM86, we need to make
> >> +	 * sure to update the mode before loading the segment descriptors so
> >> +	 * that the selectors are interpreted correctly.
> >> +	 *
> >> +	 * Need to get it to the vcpu struct immediately because it influences
> >> +	 * the CPL which is checked when loading the segment descriptors.
> > This is true only for VMX. SVM does not look at rflags. May be instead
> > of adding new x86_emulate_ops callback we need to get rid of get_cpl()
> > one and implement CPL checking using emulate.c:get_segment_selector().
> 
> The selector isn't enough for VM86. In most cases
> ctxt->ops->get_segment() would work (assuming that the dpl field is
> valid there), but it doesn't in task switches before switching the code
> segment, which already needs to have the new CPL applied to succeed.
> 
> So the only other way I could think of is to pass a flag to
> load_segment_descriptor() which says that it shouldn't check any privileges.
> 
What I mean it to have similar logic to what we have in
__vmx_get_cpl(). Something like this:

static int get_cpl(struct x86_emulate_ctxt *ctxt)
{
	if (ctxt->mode == X86EMUL_MODE_REAL)
                return 0;

        if (ctxt->mode != X86EMUL_MODE_PROT64
            && (ctx->eflags & EFLG_VM)) /* if virtual 8086 */
                return 3;

        return get_segment_selector(ctx, VCPU_SREG_CS) & 3;
}

--
			Gleb.