x86: Support compiling with indirect branch thunks

Use -mindirect-branch=thunk-extern/-mindirect-branch-register when available.
To begin with, use the retpoline thunk.  Later work will add alternative
thunks which can be selected at boot time.

This is part of XSA-254.

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

xen/arch/x86/Makefile

@@ -36,6 +36,7 @@ obj-y += io_apic.o
 obj-$(CONFIG_LIVEPATCH) += alternative.o livepatch.o
 obj-y += msi.o
 obj-y += msr.o
+obj-$(CONFIG_INDIRECT_THUNK) += indirect-thunk.o
 obj-y += ioport_emulate.o
 obj-y += irq.o
 obj-$(CONFIG_KEXEC) += machine_kexec.o

xen/arch/x86/Rules.mk

@@ -34,3 +34,10 @@ CFLAGS += -mno-sse $(call cc-option,$(CC),-mskip-rax-setup)
 ifneq ($(call cc-option,$(CC),-fvisibility=hidden,n),n)
 CFLAGS += -DGCC_HAS_VISIBILITY_ATTRIBUTE
 endif
+
+# Compile with thunk-extern, indirect-branch-register if avaiable.
+ifneq ($(call cc-option,$(CC),-mindirect-branch-register,n),n)
+CFLAGS += -mindirect-branch=thunk-extern -mindirect-branch-register
+CFLAGS += -DCONFIG_INDIRECT_THUNK
+export CONFIG_INDIRECT_THUNK=y
+endif

xen/arch/x86/indirect-thunk.S

@@ -0,0 +1,38 @@
+/*
+ * Implement __x86_indirect_thunk_* symbols for use with compatbile compilers
+ * and the -mindirect-branch=thunk-extern -mindirect-branch-register options.
+ *
+ * Copyright (c) 2017-2018 Citrix Systems Ltd.
+ *
+ * This source code is licensed under the GNU General Public License,
+ * Version 2.  See the file COPYING for more details.
+ */
+        .file __FILE__
+
+#include <asm/asm_defns.h>
+
+.macro IND_THUNK_RETPOLINE reg:req
+        call 2f
+1:
+        lfence
+        jmp 1b
+2:
+        mov %\reg, (%rsp)
+        ret
+.endm
+
+/*
+ * Build the __x86_indirect_thunk_* symbols.  Currently implement the
+ * retpoline thunk only.
+ */
+.macro GEN_INDIRECT_THUNK reg:req
+        .section .text.__x86_indirect_thunk_\reg, "ax", @progbits
+
+ENTRY(__x86_indirect_thunk_\reg)
+        IND_THUNK_RETPOLINE \reg
+.endm
+
+/* Instantiate GEN_INDIRECT_THUNK for each register except %rsp. */
+.irp reg, ax, cx, dx, bx, bp, si, di, 8, 9, 10, 11, 12, 13, 14, 15
+        GEN_INDIRECT_THUNK reg=r\reg
+.endr

xen/arch/x86/xen.lds.S

@@ -59,6 +59,7 @@ SECTIONS
   .text : {
         _stext = .;            /* Text and read-only data */
        *(.text)
+       *(.text.__x86_indirect_thunk_*)
        *(.text.cold)
        *(.text.unlikely)
        *(.fixup)