[PATCH 0/2] Fix GDT limit corruption on Intel

[PATCH 0/2] Fix GDT limit corruption on Intel

[Avi Kivity]

Intel vmx does not restore GDT.LIMIT; this allows host userspace to look at
some host kernel bits by loading a segment register and looking whether a
trap happened or not.

Fix that by reloading GDT on heavyweight exits.

Avi Kivity (2):
  KVM: VMX: Fix host GDT.LIMIT corruption
  KVM: VMX: Use host_gdt variable wherever we need the host gdt

 arch/x86/kvm/vmx.c |   19 ++++++++++---------
 1 files changed, 10 insertions(+), 9 deletions(-)

[PATCH 1/2] KVM: VMX: Fix host GDT.LIMIT corruption

[Avi Kivity]

vmx does not restore GDT.LIMIT to the host value, instead it sets it to 64KB.
This means host userspace can learn a few bits of host memory.

Fix by reloading GDTR when we load other host state.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/kvm/vmx.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 2fdcc98..27a0222 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -185,6 +185,7 @@ static void kvm_cpu_vmxoff(void);
 static DEFINE_PER_CPU(struct vmcs *, vmxarea);
 static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
 static DEFINE_PER_CPU(struct list_head, vcpus_on_cpu);
+static DEFINE_PER_CPU(struct desc_ptr, host_gdt);
 
 static unsigned long *vmx_io_bitmap_a;
 static unsigned long *vmx_io_bitmap_b;
@@ -871,6 +872,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 #endif
 	if (current_thread_info()->status & TS_USEDFPU)
 		clts();
+	load_gdt(&__get_cpu_var(host_gdt));
 }
 
 static void vmx_load_host_state(struct vcpu_vmx *vmx)
@@ -1379,6 +1381,8 @@ static int hardware_enable(void *garbage)
 		ept_sync_global();
 	}
 
+	store_gdt(&__get_cpu_var(host_gdt));
+
 	return 0;
 }
 
-- 
1.7.1

[PATCH 2/2] KVM: VMX: Use host_gdt variable wherever we need the host gdt

[Avi Kivity]

Now that we have the host gdt conveniently stored in a variable, make use
of it instead of querying the cpu.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
 arch/x86/kvm/vmx.c |   15 ++++++---------
 1 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 27a0222..cf56462 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -706,11 +706,10 @@ static void reload_tss(void)
 	/*
 	 * VT restores TR but not its size.  Useless.
 	 */
-	struct desc_ptr gdt;
+	struct desc_ptr *gdt = &__get_cpu_var(host_gdt);
 	struct desc_struct *descs;
 
-	native_store_gdt(&gdt);
-	descs = (void *)gdt.address;
+	descs = (void *)gdt->address;
 	descs[GDT_ENTRY_TSS].type = 9; /* available TSS */
 	load_TR_desc();
 }
@@ -753,7 +752,7 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 
 static unsigned long segment_base(u16 selector)
 {
-	struct desc_ptr gdt;
+	struct desc_ptr *gdt = &__get_cpu_var(host_gdt);
 	struct desc_struct *d;
 	unsigned long table_base;
 	unsigned long v;
@@ -761,8 +760,7 @@ static unsigned long segment_base(u16 selector)
 	if (!(selector & ~3))
 		return 0;
 
-	native_store_gdt(&gdt);
-	table_base = gdt.address;
+	table_base = gdt->address;
 
 	if (selector & 4) {           /* from ldt */
 		u16 ldt_selector = kvm_read_ldt();
@@ -903,7 +901,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	}
 
 	if (vcpu->cpu != cpu) {
-		struct desc_ptr dt;
+		struct desc_ptr *gdt = &__get_cpu_var(host_gdt);
 		unsigned long sysenter_esp;
 
 		kvm_migrate_timers(vcpu);
@@ -919,8 +917,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		 * processors.
 		 */
 		vmcs_writel(HOST_TR_BASE, kvm_read_tr_base()); /* 22.2.4 */
-		native_store_gdt(&dt);
-		vmcs_writel(HOST_GDTR_BASE, dt.address);   /* 22.2.4 */
+		vmcs_writel(HOST_GDTR_BASE, gdt->address);   /* 22.2.4 */
 
 		rdmsrl(MSR_IA32_SYSENTER_ESP, sysenter_esp);
 		vmcs_writel(HOST_IA32_SYSENTER_ESP, sysenter_esp); /* 22.2.3 */
-- 
1.7.1

Re: [PATCH 0/2] Fix GDT limit corruption on Intel

[Marcelo Tosatti]

On Mon, Jul 26, 2010 at 06:32:37PM +0300, Avi Kivity wrote:
> Intel vmx does not restore GDT.LIMIT; this allows host userspace to look at
> some host kernel bits by loading a segment register and looking whether a
> trap happened or not.
> 
> Fix that by reloading GDT on heavyweight exits.
> 
> Avi Kivity (2):
>   KVM: VMX: Fix host GDT.LIMIT corruption
>   KVM: VMX: Use host_gdt variable wherever we need the host gdt
> 
>  arch/x86/kvm/vmx.c |   19 ++++++++++---------
>  1 files changed, 10 insertions(+), 9 deletions(-)

Applied, thanks.