• Roman Pen's avatar
    KVM: SVM: do not zero out segment attributes if segment is unusable or not present · d9c1b543
    Roman Pen authored
    This is a fix for the problem [1], where VMCB.CPL was set to 0 and interrupt
    was taken on userspace stack.  The root cause lies in the specific AMD CPU
    behaviour which manifests itself as unusable segment attributes on SYSRET.
    The corresponding work around for the kernel is the following:
    
    61f01dd9
    
     ("x86_64, asm: Work around AMD SYSRET SS descriptor attribute issue")
    
    In other turn virtualization side treated unusable segment incorrectly and
    restored CPL from SS attributes, which were zeroed out few lines above.
    
    In current patch it is assured only that P bit is cleared in VMCB.save state
    and segment attributes are not zeroed out if segment is not presented or is
    unusable, therefore CPL can be safely restored from DPL field.
    
    This is only one part of the fix, since QEMU side should be fixed accordingly
    not to zero out attributes on its side.  Corresponding patch will follow.
    
    [1] Message id: CAJrWOzD6Xq==b-zYCDdFLgSRMPM-NkNuTSDFEtX=7MreT45i7Q@mail.gmail.com
    
    Signed-off-by: default avatarRoman Pen <roman.penyaev@profitbricks.com>
    Signed-off-by: default avatarMikhail Sennikovskii <mikhail.sennikovskii@profitbricks.com>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Cc: Radim KrÄmář <rkrcmar@redhat.com>
    Cc: kvm@vger.kernel.org
    Cc: linux-kernel@vger.kernel.org
    Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
    d9c1b543