[xen stable-4.16] x86/spec-ctrl: Use IST RSB protection for !SVM systems

[patchbot@xxxxxxx]

commit 4e351880f68e50fe1479c36a30e0e4e03d160c34
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Aug 15 15:31:49 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Aug 15 15:31:49 2022 +0200

    x86/spec-ctrl: Use IST RSB protection for !SVM systems
    
    There is a corner case where a VT-x guest which manages to reliably trigger
    non-fatal #MC's could evade the rogue RSB speculation protections that were
    supposed to be in place.
    
    This is a lack of defence in depth; Xen does not architecturally execute 
more
    RET than CALL instructions, so an attacker would have to locate a different
    gadget (e.g. SpectreRSB) first to execute a transient path of excess RET
    instructions.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: e570e8d520ab542d8d35666b95cb3a0125b7b110
    master date: 2022-08-05 12:16:24 +0100
---
 xen/arch/x86/spec_ctrl.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 864233f4cf..50660f3e23 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -1326,8 +1326,24 @@ void __init init_speculation_mitigations(void)
      * mappings.
      */
     if ( opt_rsb_hvm )
+    {
         setup_force_cpu_cap(X86_FEATURE_SC_RSB_HVM);
 
+        /*
+         * For SVM, Xen's RSB safety actions are performed before STGI, so
+         * behave atomically with respect to IST sources.
+         *
+         * For VT-x, NMIs are atomic with VMExit (the NMI gets queued but not
+         * delivered) whereas other IST sources are not atomic.  Specifically,
+         * #MC can hit ahead the RSB safety action in the vmexit path.
+         *
+         * Therefore, it is necessary for the IST logic to protect Xen against
+         * possible rogue RSB speculation.
+         */
+        if ( !cpu_has_svm )
+            default_spec_ctrl_flags |= SCF_ist_rsb;
+    }
+
     ibpb_calculations();
 
     /* Check whether Eager FPU should be enabled by default. */
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.16