KVM: x86: get CPL from SS.DPL

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>

1 files changed, 14 insertions, 21 deletions

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 0b7d58d0c5fb..ec8366c5cfea 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1338,21 +1338,6 @@ static void svm_vcpu_put(struct kvm_vcpu *vcpu)
 		wrmsrl(host_save_user_msrs[i], svm->host_user_msrs[i]);
 }
 
-static void svm_update_cpl(struct kvm_vcpu *vcpu)
-{
-	struct vcpu_svm *svm = to_svm(vcpu);
-	int cpl;
-
-	if (!is_protmode(vcpu))
-		cpl = 0;
-	else if (svm->vmcb->save.rflags & X86_EFLAGS_VM)
-		cpl = 3;
-	else
-		cpl = svm->vmcb->save.cs.selector & 0x3;
-
-	svm->vmcb->save.cpl = cpl;
-}
-
 static unsigned long svm_get_rflags(struct kvm_vcpu *vcpu)
 {
 	return to_svm(vcpu)->vmcb->save.rflags;
@@ -1360,11 +1345,12 @@ static unsigned long svm_get_rflags(struct kvm_vcpu *vcpu)
 
 static void svm_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
 {
-	unsigned long old_rflags = to_svm(vcpu)->vmcb->save.rflags;
-
+       /*
+        * Any change of EFLAGS.VM is accompained by a reload of SS
+        * (caused by either a task switch or an inter-privilege IRET),
+        * so we do not need to update the CPL here.
+        */
 	to_svm(vcpu)->vmcb->save.rflags = rflags;
-	if ((old_rflags ^ rflags) & X86_EFLAGS_VM)
-		svm_update_cpl(vcpu);
 }
 
 static void svm_cache_reg(struct kvm_vcpu *vcpu, enum kvm_reg reg)
@@ -1631,8 +1617,15 @@ static void svm_set_segment(struct kvm_vcpu *vcpu,
 		s->attrib |= (var->db & 1) << SVM_SELECTOR_DB_SHIFT;
 		s->attrib |= (var->g & 1) << SVM_SELECTOR_G_SHIFT;
 	}
-	if (seg == VCPU_SREG_CS)
-		svm_update_cpl(vcpu);
+
+	/*
+	 * This is always accurate, except if SYSRET returned to a segment
+	 * with SS.DPL != 3.  Intel does not have this quirk, and always
+	 * forces SS.DPL to 3 on sysret, so we ignore that case; fixing it
+	 * 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;
 
 	mark_dirty(svm->vmcb, VMCB_SEG);
 }