Re: [PATCH v3 0/7] KVM: few more SMM fixes

[Sean Christopherson]

On Wed, Sep 22, 2021, Paolo Bonzini wrote:
> On 22/09/21 16:46, Sean Christopherson wrote:
> > On Wed, Sep 22, 2021, Paolo Bonzini wrote:
> > > On 13/09/21 16:09, Maxim Levitsky wrote:
> > > >     KVM: x86: nVMX: re-evaluate emulation_required on nested VM exit
> > 
> > ...
> > > Queued, thanks.  However, I'm keeping patch 1 for 5.16 only.
> > 
> > I'm pretty sure the above patch is wrong, emulation_required can simply be
> > cleared on emulated VM-Exit.
> 
> Are you sure?

Pretty sure, but not 100% sure :-)

> I think you can at least set the host segment fields to a data segment that
> requires emulation.  For example the DPL of the host DS is hardcoded to zero,
> but the RPL comes from the selector field and the DS selector is not
> validated.

HOST_DS_SEL is validated:

  In the selector field for each of CS, SS, DS, ES, FS, GS and TR, the RPL
  (bits 1:0) and the TI flag (bit 2) must be 0.

> Therefore a subsequent vmentry could fail the access rights tests of 26.3.1.2
> Checks on Guest Segment Registers:

Yes, but this path is loading host state on VM-Exit.

> DS, ES, FS, GS. The DPL cannot be less than the RPL in the selector field if
> (1) the “unrestricted guest” VM-execution control is 0; (2) the register is
> usable; and (3) the Type in the access-rights field is in the range 0 – 11
> (data segment or non-conforming code segment).
> 
> Paolo
>