KVM: Fix fs/gs reload oops with invalid ldt

kvm reloads the host's fs and gs blindly, however the underlying segment
descriptors may be invalid due to the user modifying the ldt after loading
them.

Fix by using the safe accessors (loadsegment() and load_gs_index()) instead
of home grown unsafe versions.

This is CVE-2010-3698.

KVM-Stable-Tag.
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

1 files changed, 9 insertions, 15 deletions

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 49b25eee25ac..7bddfab12013 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -803,7 +803,7 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
          */
         vmx->host_state.ldt_sel = kvm_read_ldt();
         vmx->host_state.gs_ldt_reload_needed = vmx->host_state.ldt_sel;
-        vmx->host_state.fs_sel = kvm_read_fs();
+        savesegment(fs, vmx->host_state.fs_sel);
         if (!(vmx->host_state.fs_sel & 7)) {
                 vmcs_write16(HOST_FS_SELECTOR, vmx->host_state.fs_sel);
                 vmx->host_state.fs_reload_needed = 0;
@@ -811,7 +811,7 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
                 vmcs_write16(HOST_FS_SELECTOR, 0);
                 vmx->host_state.fs_reload_needed = 1;
         }
-        vmx->host_state.gs_sel = kvm_read_gs();
+        savesegment(gs, vmx->host_state.gs_sel);
         if (!(vmx->host_state.gs_sel & 7))
                 vmcs_write16(HOST_GS_SELECTOR, vmx->host_state.gs_sel);
         else {
@@ -841,27 +841,21 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 
 static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 {
-        unsigned long flags;
-
         if (!vmx->host_state.loaded)
                 return;
 
         ++vmx->vcpu.stat.host_state_reload;
         vmx->host_state.loaded = 0;
         if (vmx->host_state.fs_reload_needed)
-                kvm_load_fs(vmx->host_state.fs_sel);
+                loadsegment(fs, vmx->host_state.fs_sel);
         if (vmx->host_state.gs_ldt_reload_needed) {
                 kvm_load_ldt(vmx->host_state.ldt_sel);
-                /*
-                 * If we have to reload gs, we must take care to
-                 * preserve our gs base.
-                 */
-                local_irq_save(flags);
-                kvm_load_gs(vmx->host_state.gs_sel);
 #ifdef CONFIG_X86_64
-                wrmsrl(MSR_GS_BASE, vmcs_readl(HOST_GS_BASE));
+                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
-                local_irq_restore(flags);
         }
         reload_tss();
 #ifdef CONFIG_X86_64
@@ -2589,8 +2583,8 @@ static int vmx_vcpu_setup(struct vcpu_vmx *vmx)
         vmcs_write16(HOST_CS_SELECTOR, __KERNEL_CS);  /* 22.2.4 */
         vmcs_write16(HOST_DS_SELECTOR, __KERNEL_DS);  /* 22.2.4 */
         vmcs_write16(HOST_ES_SELECTOR, __KERNEL_DS);  /* 22.2.4 */
-        vmcs_write16(HOST_FS_SELECTOR, kvm_read_fs());    /* 22.2.4 */
-        vmcs_write16(HOST_GS_SELECTOR, kvm_read_gs());    /* 22.2.4 */
+        vmcs_write16(HOST_FS_SELECTOR, 0);            /* 22.2.4 */
+        vmcs_write16(HOST_GS_SELECTOR, 0);            /* 22.2.4 */
         vmcs_write16(HOST_SS_SELECTOR, __KERNEL_DS);  /* 22.2.4 */
 #ifdef CONFIG_X86_64
         rdmsrl(MSR_FS_BASE, a);