[xen staging] x86/spec-ctrl: Disable retpolines with CET-IBT

[patchbot@xxxxxxx]

commit 6e3f36387de566b09aa4145ea0e3bfe4814d68b4
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Feb 28 19:26:37 2022 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Mar 1 12:34:46 2022 +0000

    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@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/spec_ctrl.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 2d4cf5c7ef..12bf1c5722 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -944,10 +944,11 @@ 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.
      */
-    if ( cpu_has_xen_shstk &&
+    if ( (read_cr4() & X86_CR4_CET) &&
          (opt_thunk == THUNK_DEFAULT || opt_thunk == THUNK_RETPOLINE) )
         thunk = THUNK_JMP;
 
--
generated by git-patchbot for /home/xen/git/xen.git#staging