[PATCH v2 3/3] x86/Kconfig: introduce option to select retpoline usage

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

Add a new Kconfig option under the "Speculative hardening" section
that allows selecting whether to enable retpoline. This depends on the
underlying compiler having retpoline support.

Requested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes since v1:
 - Fix description of option to use indirect branches instead of
   indirect calls.
---
 xen/arch/x86/Kconfig |  4 ----
 xen/arch/x86/arch.mk |  2 ++
 xen/common/Kconfig   | 16 ++++++++++++++++
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 2fa456292b..7c73802adc 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -38,10 +38,6 @@ config GCC_INDIRECT_THUNK
 config CLANG_INDIRECT_THUNK
        def_bool $(cc-option,-mretpoline-external-thunk)
 
-config INDIRECT_THUNK
-       def_bool y
-       depends on GCC_INDIRECT_THUNK || CLANG_INDIRECT_THUNK
-
 config HAS_AS_CET_SS
        # binutils >= 2.29 or LLVM >= 6
        def_bool $(as-instr,wrssq %rax$(comma)0;setssbsy)
diff --git a/xen/arch/x86/arch.mk b/xen/arch/x86/arch.mk
index f2aa2a515f..0597e714f9 100644
--- a/xen/arch/x86/arch.mk
+++ b/xen/arch/x86/arch.mk
@@ -42,6 +42,7 @@ CFLAGS += -mno-red-zone -fpic
 # SSE setup for variadic function calls.
 CFLAGS += -mno-sse $(call cc-option,$(CC),-mskip-rax-setup)
 
+ifeq ($(CONFIG_INDIRECT_THUNK),y)
 # Compile with gcc thunk-extern, indirect-branch-register if available.
 CFLAGS-$(CONFIG_GCC_INDIRECT_THUNK) += -mindirect-branch=thunk-extern
 CFLAGS-$(CONFIG_GCC_INDIRECT_THUNK) += -mindirect-branch-register
@@ -49,6 +50,7 @@ CFLAGS-$(CONFIG_GCC_INDIRECT_THUNK) += -fno-jump-tables
 
 # Enable clang retpoline support if available.
 CFLAGS-$(CONFIG_CLANG_INDIRECT_THUNK) += -mretpoline-external-thunk
+endif
 
 # If supported by the compiler, reduce stack alignment to 8 bytes. But allow
 # this to be overridden elsewhere.
diff --git a/xen/common/Kconfig b/xen/common/Kconfig
index db687b1785..e688e45513 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -146,6 +146,22 @@ config SPECULATIVE_HARDEN_GUEST_ACCESS
 
          If unsure, say Y.
 
+config INDIRECT_THUNK
+       bool "Speculative Branch Target Injection Protection"
+       depends on X86 && (GCC_INDIRECT_THUNK || CLANG_INDIRECT_THUNK)
+       default y
+       help
+         Contemporary processors may use speculative execution as a
+         performance optimisation, but this can potentially be abused by an
+         attacker to leak data via speculative sidechannels.
+
+         One source of data leakage is via branch target injection.
+
+         When enabled, indirect branches are implemented using a new construct
+         called "retpoline" that prevents speculation.
+
+         If unsure, say Y.
+
 endmenu
 
 config HYPFS
-- 
2.34.1