[xen staging-4.15] x86/spec-ctrl: Disable retpolines with CET-IBT

[patchbot@xxxxxxx]

commit 67fa16fbc29ebc127372450fcb09a022269f556f
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Feb 28 19:26:37 2022 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Mar 25 17:10:38 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>
    (cherry picked from commit 6e3f36387de566b09aa4145ea0e3bfe4814d68b4)
---
 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 7ded6ecba1..1e226102d3 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -919,13 +919,14 @@ 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");
--
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