[xen staging] x86/Kconfig: introduce option to select retpoline usage

[patchbot@xxxxxxx]

commit 95d9ab46143685f169f636cfdd7997e2fc630e86
Author:     Roger Pau Monne <roger.pau@xxxxxxxxxx>
AuthorDate: Fri Feb 18 15:34:16 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Feb 21 18:17:56 2022 +0000

    x86/Kconfig: introduce option to select retpoline usage
    
    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>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/Kconfig |  4 ----
 xen/common/Kconfig   | 14 ++++++++++++++
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 1465874097..41198b0f96 100644
--- a/xen/arch/x86/Kconfig
+++ b/xen/arch/x86/Kconfig
@@ -36,10 +36,6 @@ config CC_HAS_INDIRECT_THUNK
        def_bool $(cc-option,-mindirect-branch-register) || \
                 $(cc-option,-mretpoline-external-thunk)
 
-config INDIRECT_THUNK
-       def_bool y
-       depends on CC_HAS_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/common/Kconfig b/xen/common/Kconfig
index db687b1785..6443943889 100644
--- a/xen/common/Kconfig
+++ b/xen/common/Kconfig
@@ -85,6 +85,20 @@ config STATIC_MEMORY
 
 menu "Speculative hardening"
 
+config INDIRECT_THUNK
+       bool "Speculative Branch Target Injection Protection"
+       depends on CC_HAS_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.
+
 config SPECULATIVE_HARDEN_ARRAY
        bool "Speculative Array Hardening"
        default y
--
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