Re: [PATCH v2 4/4] KVM: x86: get CPL from SS.DPL

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, May 15, 2014 at 06:51:31PM +0200, Paolo Bonzini wrote:
> CS.RPL is not equal to the CPL in the few instructions between
> setting CR0.PE and reloading CS.  And CS.DPL is also not equal
> to the CPL for conforming code segments.
> 
> However, SS.DPL *is* always equal to the CPL except for the weird
> case of SYSRET on AMD processors, which sets SS.DPL=SS.RPL from the
> value in the STAR MSR, but force CPL=3 (Intel instead forces
> SS.DPL=SS.RPL=CPL=3).
> 
> So this patch:
> 
> - modifies SVM to update the CPL from SS.DPL rather than CS.RPL;
> the above case with SYSRET is not broken further, and the way
> to fix it would be to pass the CPL to userspace and back
> 
> - modifies VMX to always return the CPL from SS.DPL (except
> forcing it to 0 if we are emulating real mode via vm86 mode;
> in vm86 mode all DPLs have to be 3, but real mode does allow
> privileged instructions).  It also removes the CPL cache,
> which becomes a duplicate of the SS access rights cache.
> 
> This fixes doing KVM_IOCTL_SET_SREGS exactly after setting
> CR0.PE=1 but before CS has been reloaded.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>


Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>