[Hardy] KVM: Fix fs/gs reload oops with invalid ldt

[Stefan Bader]

From: Avi Kivity <avi at redhat.com>

CVE-2010-3698

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 at redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti at redhat.com>
(backported from commit 9581d442b9058d3699b4be568b6e5eae38a41493 upstream)
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 arch/x86/kvm/svm.c         |   15 ++++++++++-----
 arch/x86/kvm/vmx.c         |   24 +++++++++---------------
 include/asm-x86/kvm_host.h |   30 +++++++-----------------------
 3 files changed, 26 insertions(+), 43 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index a24f2e3..ac9a98c 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -1608,8 +1608,8 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	pre_svm_run(svm);
 
 	save_host_msrs(vcpu);
-	fs_selector = read_fs();
-	gs_selector = read_gs();
+	savesegment(fs, fs_selector);
+	savesegment(gs, gs_selector);
 	ldt_selector = read_ldt();
 	svm->host_cr2 = kvm_read_cr2();
 	svm->host_dr6 = read_dr6();
@@ -1743,10 +1743,15 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
 	write_dr7(svm->host_dr7);
 	kvm_write_cr2(svm->host_cr2);
 
-	load_fs(fs_selector);
-	load_gs(gs_selector);
-	load_ldt(ldt_selector);
 	load_host_msrs(vcpu);
+	loadsegment(fs, fs_selector);
+#ifdef CONFIG_X86_64
+	load_gs_index(gs_selector);
+	wrmsrl(MSR_KERNEL_GS_BASE, current->thread.gs);
+#else
+	loadsegment(gs, gs_selector);
+#endif
+	load_ldt(ldt_selector);
 
 	reload_tss(vcpu);
 
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index fcfd55b..54e5d9f 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -448,7 +448,7 @@ static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 	 */
 	vmx->host_state.ldt_sel = read_ldt();
 	vmx->host_state.gs_ldt_reload_needed = vmx->host_state.ldt_sel;
-	vmx->host_state.fs_sel = 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;
@@ -456,7 +456,7 @@ 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 = 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 {
@@ -484,27 +484,21 @@ 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)
-		load_fs(vmx->host_state.fs_sel);
+		loadsegment(fs, vmx->host_state.fs_sel);
 	if (vmx->host_state.gs_ldt_reload_needed) {
 		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);
-		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();
 	save_msrs(vmx->guest_msrs, vmx->save_nmsrs);
@@ -1622,8 +1616,8 @@ 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, read_fs());    /* 22.2.4 */
-	vmcs_write16(HOST_GS_SELECTOR, 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);
diff --git a/include/asm-x86/kvm_host.h b/include/asm-x86/kvm_host.h
index 6d2d0b7..88cee4c 100644
--- a/include/asm-x86/kvm_host.h
+++ b/include/asm-x86/kvm_host.h
@@ -530,20 +530,6 @@ static inline struct kvm_mmu_page *page_header(hpa_t shadow_page)
 	return (struct kvm_mmu_page *)page_private(page);
 }
 
-static inline u16 read_fs(void)
-{
-	u16 seg;
-	asm("mov %%fs, %0" : "=g"(seg));
-	return seg;
-}
-
-static inline u16 read_gs(void)
-{
-	u16 seg;
-	asm("mov %%gs, %0" : "=g"(seg));
-	return seg;
-}
-
 static inline u16 read_ldt(void)
 {
 	u16 ldt;
@@ -551,15 +537,13 @@ static inline u16 read_ldt(void)
 	return ldt;
 }
 
-static inline void load_fs(u16 sel)
-{
-	asm("mov %0, %%fs" : : "rm"(sel));
-}
-
-static inline void load_gs(u16 sel)
-{
-	asm("mov %0, %%gs" : : "rm"(sel));
-}
+/*
+ * Save a segment register away
+ */
+#ifndef savesegment
+#define savesegment(seg, value)					\
+	asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
+#endif
 
 #ifndef load_ldt
 static inline void load_ldt(u16 sel)
-- 
1.7.0.4