[PATCH] KVM: VMX: Fix host userspace gsbase corruption

[PATCH] KVM: VMX: Fix host userspace gsbase corruption

[Avi Kivity]

We now use load_gs_index() to load gs safely; unfortunately this also
changes MSR_KERNEL_GS_BASE, which we managed separately.  This resulted
in confusion and breakage running 32-bit host userspace on a 64-bit kernel.

Fix by
- saving guest MSR_KERNEL_GS_BASE before we we reload the host's gs
- doing the host save/load unconditionally, instead of only when in guest
  long mode

Things can be cleaned up further, but this is the minmal fix for now.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/kvm/vmx.c |   15 +++++++--------
 1 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 9367abc..0badeac 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -821,10 +821,9 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 #endif
 
 #ifdef CONFIG_X86_64
-	if (is_long_mode(&vmx->vcpu)) {
-		rdmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
+	rdmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
+	if (is_long_mode(&vmx->vcpu))
 		wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
-	}
 #endif
 	for (i = 0; i < vmx->save_nmsrs; ++i)
 		kvm_set_shared_msr(vmx->guest_msrs[i].index,
@@ -839,11 +838,14 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 
 	++vmx->vcpu.stat.host_state_reload;
 	vmx->host_state.loaded = 0;
+#ifdef CONFIG_X86_64
+	if (is_long_mode(&vmx->vcpu))
+		rdmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
+#endif
 	if (vmx->host_state.gs_ldt_reload_needed) {
 		kvm_load_ldt(vmx->host_state.ldt_sel);
 #ifdef CONFIG_X86_64
 		load_gs_index(vmx->host_state.gs_sel);
-		wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gs);
 #else
 		loadsegment(gs, vmx->host_state.gs_sel);
 #endif
@@ -852,10 +854,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 		loadsegment(fs, vmx->host_state.fs_sel);
 	reload_tss();
 #ifdef CONFIG_X86_64
-	if (is_long_mode(&vmx->vcpu)) {
-		rdmsrl(MSR_KERNEL_GS_BASE, vmx->msr_guest_kernel_gs_base);
-		wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
-	}
+	wrmsrl(MSR_KERNEL_GS_BASE, vmx->msr_host_kernel_gs_base);
 #endif
 	if (current_thread_info()->status & TS_USEDFPU)
 		clts();
-- 
1.7.3.1

Re: [PATCH] KVM: VMX: Fix host userspace gsbase corruption

[Marcelo Tosatti]

On Thu, Nov 11, 2010 at 12:37:26PM +0200, Avi Kivity wrote:
> We now use load_gs_index() to load gs safely; unfortunately this also
> changes MSR_KERNEL_GS_BASE, which we managed separately.  This resulted
> in confusion and breakage running 32-bit host userspace on a 64-bit kernel.
> 
> Fix by
> - saving guest MSR_KERNEL_GS_BASE before we we reload the host's gs
> - doing the host save/load unconditionally, instead of only when in guest
>   long mode
> 
> Things can be cleaned up further, but this is the minmal fix for now.
> 
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
>  arch/x86/kvm/vmx.c |   15 +++++++--------
>  1 files changed, 7 insertions(+), 8 deletions(-)

Applied, thanks.