From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Mon, 28 Feb 2022 19:26:37 +0000 (+0000)
Subject: x86/spec-ctrl: Disable retpolines with CET-IBT
X-Git-Tag: archive/raspbian/4.16.1-1+rpi1^2~38^2~39
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=351428de6feee3769a0ba69fdab3f521be85e891;p=xen.git

x86/spec-ctrl: Disable retpolines with CET-IBT

CET-IBT depend on executing indirect branches for protections to apply.
Extend the clobber for CET-SS to all of CET.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 6e3f36387de566b09aa4145ea0e3bfe4814d68b4)
---

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index ae076bec3a..c19464da70 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -948,13 +948,14 @@ void __init init_speculation_mitigations(void)
                      boot_cpu_has(X86_FEATURE_IBRS));
 
     /*
-     * First, disable the use of retpolines if Xen is using shadow stacks, as
-     * they are incompatible.
+     * First, disable the use of retpolines if Xen is using CET.  Retpolines
+     * are a ROP gadget so incompatbile with Shadow Stacks, while IBT depends
+     * on executing indirect branches for the safety properties to apply.
      *
      * In the absence of retpolines, IBRS needs to be used for speculative
      * safety.  All CET-capable hardware has efficient IBRS.
      */
-    if ( cpu_has_xen_shstk )
+    if ( read_cr4() & X86_CR4_CET )
     {
         if ( !has_spec_ctrl )
             printk(XENLOG_WARNING "?!? CET active, but no MSR_SPEC_CTRL?\n");