x86: Introduce support for CET-IBT

CET Indirect Branch Tracking is a hardware feature designed to provide
forward-edge control flow integrity, protecting against jump/call oriented
programming.

IBT requires the placement of endbr{32,64} instructions at the target of every
indirect call/jmp, and every entrypoint.

However, the default -fcf-protection=branch places an endbr{32,64} on every
function which far more than necessary, and reduces the quantity of protection
afforded.  Therefore, we use manual placement using the cf_check attribute.

It is necessary to check for both compiler and assembler support, as the
notrack prefix can be emitted in certain cases.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

Config.mk

@@ -190,7 +190,6 @@ APPEND_CFLAGS += $(foreach i, $(APPEND_INCLUDES), -I$(i))
 
 EMBEDDED_EXTRA_CFLAGS := -nopie -fno-stack-protector -fno-stack-protector-all
 EMBEDDED_EXTRA_CFLAGS += -fno-exceptions -fno-asynchronous-unwind-tables
-EMBEDDED_EXTRA_CFLAGS += -fcf-protection=none
 
 XEN_EXTFILES_URL ?= http://xenbits.xen.org/xen-extfiles
 # All the files at that location were downloaded from elsewhere on

tools/firmware/Makefile

@@ -6,6 +6,8 @@ TARGET      := hvmloader/hvmloader
 INST_DIR := $(DESTDIR)$(XENFIRMWAREDIR)
 DEBG_DIR := $(DESTDIR)$(DEBUG_DIR)$(XENFIRMWAREDIR)
 
+EMBEDDED_EXTRA_CFLAGS += -fcf-protection=none
+
 SUBDIRS-y :=
 SUBDIRS-$(CONFIG_OVMF) += ovmf-dir
 SUBDIRS-$(CONFIG_SEABIOS) += seabios-dir

tools/libs/guest/xg_dom_decompress_unsafe.h

@@ -8,6 +8,8 @@ typedef int decompress_fn(unsigned char *inbuf, unsigned int len,
                           void (*error)(const char *x));
 #endif
 
+#define cf_check /* No Control Flow Integriy checking */
+
 int xc_dom_decompress_unsafe(
     decompress_fn fn, struct xc_dom_image *dom, void **blob, size_t *size)
     __attribute__((visibility("internal")));

tools/tests/x86_emulator/x86-emulate.h

@@ -54,6 +54,8 @@
 #define likely(x)   __builtin_expect(!!(x), true)
 #define unlikely(x) __builtin_expect(!!(x), false)
 
+#define cf_check /* No Control Flow Integriy checking */
+
 #define container_of(ptr, type, member) ({             \
     typeof(((type *)0)->member) *mptr__ = (ptr);       \
     (type *)((char *)mptr__ - offsetof(type, member)); \

xen/arch/x86/Kconfig

@@ -40,6 +40,11 @@ config HAS_AS_CET_SS
 	# binutils >= 2.29 or LLVM >= 6
 	def_bool $(as-instr,wrssq %rax$(comma)0;setssbsy)
 
+config HAS_CC_CET_IBT
+	# GCC >= 9 and binutils >= 2.29
+	# Retpoline check to work around https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93654
+	def_bool $(cc-option,-fcf-protection=branch -mmanual-endbr -mindirect-branch=thunk-extern) && $(as-instr,endbr64)
+
 menu "Architecture Features"
 
 source "arch/Kconfig"
@@ -125,6 +130,18 @@ config XEN_SHSTK
 	  When CET-SS is active, 32bit PV guests cannot be used.  Backwards
 	  compatiblity can be provided via the PV Shim mechanism.
 
+config XEN_IBT
+	bool "Supervisor Indirect Branch Tracking"
+	depends on HAS_CC_CET_IBT
+	default y
+	help
+	  Control-flow Enforcement Technology (CET) is a set of features in
+	  hardware designed to combat Return-oriented Programming (ROP, also
+	  call/jump COP/JOP) attacks.  Indirect Branch Tracking is one CET
+	  feature designed to provide function pointer protection.
+
+	  This option arranges for Xen to use CET-IBT for its own protection.
+
 config SHADOW_PAGING
 	bool "Shadow Paging"
 	default !PV_SHIM_EXCLUSIVE

xen/arch/x86/arch.mk

@@ -52,6 +52,12 @@ CFLAGS-$(CONFIG_CC_IS_GCC) += -fno-jump-tables
 CFLAGS-$(CONFIG_CC_IS_CLANG) += -mretpoline-external-thunk
 endif
 
+ifdef CONFIG_XEN_IBT
+CFLAGS += -fcf-protection=branch -mmanual-endbr
+else
+$(call cc-option-add,CFLAGS,CC,-fcf-protection=none)
+endif
+
 # If supported by the compiler, reduce stack alignment to 8 bytes. But allow
 # this to be overridden elsewhere.
 $(call cc-option-add,CFLAGS_stack_boundary,CC,-mpreferred-stack-boundary=3)

xen/arch/x86/configs/pvshim_defconfig

@@ -8,6 +8,7 @@ CONFIG_NR_CPUS=32
 CONFIG_EXPERT=y
 # Disable features not used by the PV shim
 # CONFIG_XEN_SHSTK is not set
+# CONFIG_XEN_IBT is not set
 # CONFIG_GRANT_TABLE is not set
 # CONFIG_HYPFS is not set
 # CONFIG_BIGMEM is not set

xen/arch/x86/include/asm/asm-defns.h

@@ -57,6 +57,12 @@
     INDIRECT_BRANCH jmp \arg
 .endm
 
+#ifdef CONFIG_XEN_IBT
+# define ENDBR64 endbr64
+#else
+# define ENDBR64
+#endif
+
 .macro guest_access_mask_ptr ptr:req, scratch1:req, scratch2:req
 #if defined(CONFIG_SPECULATIVE_HARDEN_GUEST_ACCESS)
     /*

xen/arch/x86/include/asm/cpufeature.h

@@ -152,6 +152,7 @@
 #define cpu_has_nscb            boot_cpu_has(X86_FEATURE_NSCB)
 #define cpu_has_xen_lbr         boot_cpu_has(X86_FEATURE_XEN_LBR)
 #define cpu_has_xen_shstk       boot_cpu_has(X86_FEATURE_XEN_SHSTK)
+#define cpu_has_xen_ibt         boot_cpu_has(X86_FEATURE_XEN_IBT)
 
 #define cpu_has_msr_tsc_aux     (cpu_has_rdtscp || cpu_has_rdpid)
 

xen/arch/x86/include/asm/cpufeatures.h

@@ -39,6 +39,7 @@ XEN_CPUFEATURE(SC_VERW_PV,        X86_SYNTH(23)) /* VERW used by Xen for PV */
 XEN_CPUFEATURE(SC_VERW_HVM,       X86_SYNTH(24)) /* VERW used by Xen for HVM */
 XEN_CPUFEATURE(SC_VERW_IDLE,      X86_SYNTH(25)) /* VERW used by Xen for idle */
 XEN_CPUFEATURE(XEN_SHSTK,         X86_SYNTH(26)) /* Xen uses CET Shadow Stacks */
+XEN_CPUFEATURE(XEN_IBT,           X86_SYNTH(27)) /* Xen uses CET Indirect Branch Tracking */
 
 /* Bug words follow the synthetic words. */
 #define X86_NR_BUG 1

xen/include/xen/compiler.h

@@ -37,6 +37,12 @@
 # define nocall
 #endif
 
+#ifdef CONFIG_XEN_IBT
+# define cf_check     __attribute__((__cf_check__))
+#else
+# define cf_check
+#endif
+
 #if (!defined(__clang__) && (__GNUC__ == 4) && (__GNUC_MINOR__ < 5))
 #define unreachable() do {} while (1)
 #else