[PATCH AUTOSEL for 4.4 080/167] KVM: nVMX: Fix handling of lmsw instruction

Sasha Levin
2018-03-19T17:21:47Z

From: Jan H. Schönherr <jschoenh@amazon.de>

[ Upstream commit e1d39b17e044e8ae819827810d87d809ba5f58c0 ]

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>
Reviewed-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
---
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 849517805eef..8dda4e88afc4 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -7650,11 +7650,13 @@ 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 &
@@ -7709,6 +7711,7 @@ 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;
--
2.14.1