[PATCH v1 3/3] x86: Make the GDT remapping read-only on 64 bit

[Thomas Garnier <thgarnie@google.com>]

This patch makes the GDT remapped pages read-only to prevent corruption.
This change is done only on 64 bit.

The native_load_tr_desc function was adapted to correctly handle a
read-only GDT. The LTR instruction always writes to the GDT TSS entry.
This generates a page fault if the GDT is read-only. This change checks
if the current GDT is a remap and swap GDTs as needed. This function was
tested by booting multiple machines and checking hibernation works
properly.

KVM SVM and VMX were adapted to use the writeable GDT. On VMX, the GDT
description and writeble address were put in two per-cpu variables for
convenience. For testing, VMs were started and restored on multiple
configurations.

Signed-off-by: Thomas Garnier <thgarnie@google.com>
---
Based on next-20170119
---
 arch/x86/include/asm/desc.h      | 44 +++++++++++++++++++++++++++++++++++-----
 arch/x86/include/asm/processor.h |  1 +
 arch/x86/kernel/cpu/common.c     | 36 ++++++++++++++++++++++++++------
 arch/x86/kvm/svm.c               |  5 ++---
 arch/x86/kvm/vmx.c               | 23 +++++++++++++--------
 5 files changed, 86 insertions(+), 23 deletions(-)

diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
index 12080d87da3b..6ed68d449c7b 100644
--- a/arch/x86/include/asm/desc.h
+++ b/arch/x86/include/asm/desc.h
@@ -50,6 +50,13 @@ static inline struct desc_struct *get_cpu_gdt_table(unsigned int cpu)
 	return per_cpu(gdt_page, cpu).gdt;
 }
 
+static inline struct desc_struct *get_current_gdt_table(void)
+{
+	return get_cpu_var(gdt_page).gdt;
+}
+
+struct desc_struct *get_remapped_gdt(int cpu);
+
 #ifdef CONFIG_X86_64
 
 static inline void pack_gate(gate_desc *gate, unsigned type, unsigned long func,
@@ -208,11 +215,6 @@ static inline void native_set_ldt(const void *addr, unsigned int entries)
 	}
 }
 
-static inline void native_load_tr_desc(void)
-{
-	asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
-}
-
 static inline void native_load_gdt(const struct desc_ptr *dtr)
 {
 	asm volatile("lgdt %0"::"m" (*dtr));
@@ -233,6 +235,38 @@ static inline void native_store_idt(struct desc_ptr *dtr)
 	asm volatile("sidt %0":"=m" (*dtr));
 }
 
+/*
+ * The LTR instruction marks the TSS GDT entry as busy. In 64bit, the GDT is
+ * a read-only remapping. To prevent a page fault, the GDT is switched to the
+ * original writeable version when needed.
+ */
+#ifdef CONFIG_X86_64
+static inline void native_load_tr_desc(void)
+{
+	struct desc_ptr gdt;
+	int cpu = raw_smp_processor_id();
+	bool restore = false;
+	struct desc_struct *remapped_gdt;
+
+	native_store_gdt(&gdt);
+	remapped_gdt = get_remapped_gdt(cpu);
+
+	/* Swap and restore only if the current GDT is the remap. */
+	if (gdt.address == (unsigned long)remapped_gdt) {
+		load_percpu_gdt(cpu);
+		restore = true;
+	}
+	asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
+	if (restore)
+		load_remapped_gdt(cpu);
+}
+#else
+static inline void native_load_tr_desc(void)
+{
+	asm volatile("ltr %w0"::"q" (GDT_ENTRY_TSS*8));
+}
+#endif
+
 static inline unsigned long native_store_tr(void)
 {
 	unsigned long tr;
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 280211ad8be9..b58033e991ab 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -705,6 +705,7 @@ extern struct desc_ptr		early_gdt_descr;
 
 extern void cpu_set_gdt(int);
 extern void switch_to_new_gdt(int);
+extern void load_percpu_gdt(int);
 extern void load_remapped_gdt(int);
 extern void load_percpu_segment(int);
 extern void cpu_init(void);
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 7d940b0e805a..bf1266212a40 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -443,18 +443,45 @@ void load_percpu_segment(int cpu)
 	load_stack_canary_segment();
 }
 
+/* Load the GDT from the per-cpu structure */
+void load_percpu_gdt(int cpu)
+{
+	struct desc_ptr gdt_descr;
+
+	gdt_descr.address = (long)get_cpu_gdt_table(cpu);
+	gdt_descr.size = GDT_SIZE - 1;
+	load_gdt(&gdt_descr);
+}
+EXPORT_SYMBOL(load_percpu_gdt);
+
+/* Provide the fixmap address of the remapped GDT */
+struct desc_struct *get_remapped_gdt(int cpu)
+{
+	return (struct desc_struct *)__fix_to_virt(FIX_GDT_REMAP_BEGIN + cpu);
+
+}
+EXPORT_SYMBOL(get_remapped_gdt);
+
+/* On 64bit the GDT remapping is read-only */
+#ifdef CONFIG_X86_64
+#define GDT_REMAP_PROT PAGE_KERNEL_RO
+#else
+#define GDT_REMAP_PROT PAGE_KERNEL
+#endif
+
 /* Load a fixmap remapping of the per-cpu GDT */
 void load_remapped_gdt(int cpu)
 {
 	struct desc_ptr gdt_descr;
 	unsigned long idx = FIX_GDT_REMAP_BEGIN + cpu;
 
-	__set_fixmap(idx, __pa(get_cpu_gdt_table(cpu)), PAGE_KERNEL);
+	__set_fixmap(idx, __pa(get_cpu_gdt_table(cpu)), GDT_REMAP_PROT);
 
 	gdt_descr.address = (long)__fix_to_virt(idx);
 	gdt_descr.size = GDT_SIZE - 1;
 	load_gdt(&gdt_descr);
 }
+EXPORT_SYMBOL(load_remapped_gdt);
 
 /*
  * Current gdt points %fs at the "master" per-cpu area: after this,
@@ -462,11 +489,8 @@ void load_remapped_gdt(int cpu)
  */
 void switch_to_new_gdt(int cpu)
 {
-	struct desc_ptr gdt_descr;
-
-	gdt_descr.address = (long)get_cpu_gdt_table(cpu);
-	gdt_descr.size = GDT_SIZE - 1;
-	load_gdt(&gdt_descr);
+	/* Load the original GDT */
+	load_percpu_gdt(cpu);
 	/* Reload the per-cpu base */
 	load_percpu_segment(cpu);
 }
diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index d0414f054bdf..da261f231017 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -741,7 +741,6 @@ static int svm_hardware_enable(void)
 
 	struct svm_cpu_data *sd;
 	uint64_t efer;
-	struct desc_ptr gdt_descr;
 	struct desc_struct *gdt;
 	int me = raw_smp_processor_id();
 
@@ -763,8 +762,7 @@ static int svm_hardware_enable(void)
 	sd->max_asid = cpuid_ebx(SVM_CPUID_FUNC) - 1;
 	sd->next_asid = sd->max_asid + 1;
 
-	native_store_gdt(&gdt_descr);
-	gdt = (struct desc_struct *)gdt_descr.address;
+	gdt = get_current_gdt_table();
 	sd->tss_desc = (struct kvm_ldttss_desc *)(gdt + GDT_ENTRY_TSS);
 
 	wrmsrl(MSR_EFER, efer | EFER_SVME);
@@ -4251,6 +4249,7 @@ static void reload_tss(struct kvm_vcpu *vcpu)
 
 	struct svm_cpu_data *sd = per_cpu(svm_data, cpu);
 	sd->tss_desc->type = 9; /* available 32/64-bit TSS */
+
 	load_TR_desc();
 }
 
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index d2fe3a51876c..acbf78c962d0 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -935,7 +935,8 @@ static DEFINE_PER_CPU(struct vmcs *, current_vmcs);
  * when a CPU is brought down, and we need to VMCLEAR all VMCSs loaded on it.
  */
 static DEFINE_PER_CPU(struct list_head, loaded_vmcss_on_cpu);
-static DEFINE_PER_CPU(struct desc_ptr, host_gdt);
+static DEFINE_PER_CPU(struct desc_ptr, host_gdt_desc);
+static DEFINE_PER_CPU(unsigned long, host_gdt);
 
 /*
  * We maintian a per-CPU linked-list of vCPU, so in wakeup_handler() we
@@ -1997,10 +1998,9 @@ static void reload_tss(void)
 	/*
 	 * VT restores TR but not its size.  Useless.
 	 */
-	struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
 	struct desc_struct *descs;
 
-	descs = (void *)gdt->address;
+	descs = (void *)get_cpu_var(host_gdt);
 	descs[GDT_ENTRY_TSS].type = 9; /* available TSS */
 	load_TR_desc();
 }
@@ -2061,7 +2061,6 @@ static bool update_transition_efer(struct vcpu_vmx *vmx, int efer_offset)
 
 static unsigned long segment_base(u16 selector)
 {
-	struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
 	struct desc_struct *d;
 	unsigned long table_base;
 	unsigned long v;
@@ -2069,7 +2068,7 @@ static unsigned long segment_base(u16 selector)
 	if (!(selector & ~3))
 		return 0;
 
-	table_base = gdt->address;
+	table_base = get_cpu_var(host_gdt);
 
 	if (selector & 4) {           /* from ldt */
 		u16 ldt_selector = kvm_read_ldt();
@@ -2185,7 +2184,7 @@ static void __vmx_load_host_state(struct vcpu_vmx *vmx)
 #endif
 	if (vmx->host_state.msr_host_bndcfgs)
 		wrmsrl(MSR_IA32_BNDCFGS, vmx->host_state.msr_host_bndcfgs);
-	load_gdt(this_cpu_ptr(&host_gdt));
+	load_gdt(this_cpu_ptr(&host_gdt_desc));
 }
 
 static void vmx_load_host_state(struct vcpu_vmx *vmx)
@@ -2287,7 +2286,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	}
 
 	if (!already_loaded) {
-		struct desc_ptr *gdt = this_cpu_ptr(&host_gdt);
+		unsigned long gdt = get_cpu_var(host_gdt);
 		unsigned long sysenter_esp;
 
 		kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
@@ -2297,7 +2296,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 		 * processors.
 		 */
 		vmcs_writel(HOST_TR_BASE, kvm_read_tr_base()); /* 22.2.4 */
-		vmcs_writel(HOST_GDTR_BASE, gdt->address);   /* 22.2.4 */
+		vmcs_writel(HOST_GDTR_BASE, gdt);   /* 22.2.4 */
 
 		rdmsrl(MSR_IA32_SYSENTER_ESP, sysenter_esp);
 		vmcs_writel(HOST_IA32_SYSENTER_ESP, sysenter_esp); /* 22.2.3 */
@@ -3523,7 +3522,13 @@ static int hardware_enable(void)
 		ept_sync_global();
 	}
 
-	native_store_gdt(this_cpu_ptr(&host_gdt));
+	native_store_gdt(this_cpu_ptr(&host_gdt_desc));
+
+	/*
+	 * The GDT is remapped and can be read-only, use the underlying memory
+	 * that is always writeable.
+	 */
+	per_cpu(host_gdt, cpu) = (unsigned long)get_current_gdt_table();
 
 	return 0;
 }
-- 
2.11.0.483.g087da7b7c-goog