[PATCH] KVM: nVMX: Fix handling of lmsw instruction

["Jan H. Schönherr" <jschoenh@amazon.de>]

The decision whether or not to exit from L2 to L1 on an lmsw instruction is
based on bogus values: instead of using the information encoded within the
exit qualification, it uses the data also used for the mov-to-cr
instruction, which boils down to using whatever is in %eax at that point.

Use the correct values instead.

Without this fix, an L1 may not get notified when a 32-bit Linux L2
switches its secondary CPUs to protected mode; the L1 is only notified on
the next modification of CR0. This short time window poses a problem, when
there is some other reason to exit to L1 in between. Then, L2 will be
resumed in real mode and chaos ensues.

Signed-off-by: Jan H. Schönherr <jschoenh@amazon.de>
---
 arch/x86/kvm/vmx.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index c6f4ad4..116569a 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -7913,11 +7913,13 @@ static bool nested_vmx_exit_handled_cr(struct kvm_vcpu *vcpu,
 {
 	unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
 	int cr = exit_qualification & 15;
-	int reg = (exit_qualification >> 8) & 15;
-	unsigned long val = kvm_register_readl(vcpu, reg);
+	int reg;
+	unsigned long val;
 
 	switch ((exit_qualification >> 4) & 3) {
 	case 0: /* mov to cr */
+		reg = (exit_qualification >> 8) & 15;
+		val = kvm_register_readl(vcpu, reg);
 		switch (cr) {
 		case 0:
 			if (vmcs12->cr0_guest_host_mask &
@@ -7972,6 +7974,7 @@ static bool nested_vmx_exit_handled_cr(struct kvm_vcpu *vcpu,
 		 * lmsw can change bits 1..3 of cr0, and only set bit 0 of
 		 * cr0. Other attempted changes are ignored, with no exit.
 		 */
+		val = (exit_qualification >> LMSW_SOURCE_DATA_SHIFT) & 0x0f;
 		if (vmcs12->cr0_guest_host_mask & 0xe &
 		    (val ^ vmcs12->cr0_read_shadow))
 			return true;
-- 
2.3.1.dirty