From: Roman Pen roman.penyaev@profitbricks.com
[ Upstream commit d9c1b5431d5f0e07575db785a022bce91051ac1d ]
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:
61f01dd941ba ("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: Roman Pen roman.penyaev@profitbricks.com Signed-off-by: Mikhail 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: Paolo Bonzini pbonzini@redhat.com Signed-off-by: Sasha Levin alexander.levin@microsoft.com --- arch/x86/kvm/svm.c | 24 +++++++++++------------- 1 file changed, 11 insertions(+), 13 deletions(-)
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c index 36414d13289f..2e0c64a08549 100644 --- a/arch/x86/kvm/svm.c +++ b/arch/x86/kvm/svm.c @@ -1467,6 +1467,7 @@ static void svm_get_segment(struct kvm_vcpu *vcpu, */ if (var->unusable) var->db = 0; + /* This is symmetric with svm_set_segment() */ var->dpl = to_svm(vcpu)->vmcb->save.cpl; break; } @@ -1611,18 +1612,14 @@ static void svm_set_segment(struct kvm_vcpu *vcpu, s->base = var->base; s->limit = var->limit; s->selector = var->selector; - if (var->unusable) - s->attrib = 0; - else { - s->attrib = (var->type & SVM_SELECTOR_TYPE_MASK); - s->attrib |= (var->s & 1) << SVM_SELECTOR_S_SHIFT; - s->attrib |= (var->dpl & 3) << SVM_SELECTOR_DPL_SHIFT; - s->attrib |= (var->present & 1) << SVM_SELECTOR_P_SHIFT; - s->attrib |= (var->avl & 1) << SVM_SELECTOR_AVL_SHIFT; - s->attrib |= (var->l & 1) << SVM_SELECTOR_L_SHIFT; - s->attrib |= (var->db & 1) << SVM_SELECTOR_DB_SHIFT; - s->attrib |= (var->g & 1) << SVM_SELECTOR_G_SHIFT; - } + s->attrib = (var->type & SVM_SELECTOR_TYPE_MASK); + s->attrib |= (var->s & 1) << SVM_SELECTOR_S_SHIFT; + s->attrib |= (var->dpl & 3) << SVM_SELECTOR_DPL_SHIFT; + s->attrib |= ((var->present & 1) && !var->unusable) << SVM_SELECTOR_P_SHIFT; + s->attrib |= (var->avl & 1) << SVM_SELECTOR_AVL_SHIFT; + s->attrib |= (var->l & 1) << SVM_SELECTOR_L_SHIFT; + s->attrib |= (var->db & 1) << SVM_SELECTOR_DB_SHIFT; + s->attrib |= (var->g & 1) << SVM_SELECTOR_G_SHIFT;
/* * This is always accurate, except if SYSRET returned to a segment @@ -1631,7 +1628,8 @@ static void svm_set_segment(struct kvm_vcpu *vcpu, * would entail passing the CPL to userspace and back. */ if (seg == VCPU_SREG_SS) - svm->vmcb->save.cpl = (s->attrib >> SVM_SELECTOR_DPL_SHIFT) & 3; + /* This is symmetric with svm_get_segment() */ + svm->vmcb->save.cpl = (var->dpl & 3);
mark_dirty(svm->vmcb, VMCB_SEG); }