KVM: SVM: Make sure KERNEL_GS_BASE is valid when loading gs_index
The gs_index loading code uses the swapgs instruction to
switch to the user gs_base temporarily. This is unsave in an
lightweight exit-path in KVM on AMD because the
KERNEL_GS_BASE MSR is switches lazily. An NMI happening in
the critical path of load_gs_index may use the wrong GS_BASE
value then leading to unpredictable behavior, e.g. a
triple-fault.
This patch fixes the issue by making sure that load_gs_index
is called only with a valid KERNEL_GS_BASE value loaded in
KVM.
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 25bd1bc..54ce246 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1150,8 +1150,8 @@
kvm_load_ldt(svm->host.ldt);
#ifdef CONFIG_X86_64
loadsegment(fs, svm->host.fs);
- load_gs_index(svm->host.gs);
wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gs);
+ load_gs_index(svm->host.gs);
#else
loadsegment(gs, svm->host.gs);
#endif