Mailing List Archive

commit 4e351880f68e50fe1479c36a30e0e4e03d160c34
Author: Andrew Cooper <andrew.cooper3@citrix.com>
AuthorDate: Mon Aug 15 15:31:49 2022 +0200
Commit: Jan Beulich <jbeulich@suse.com>
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@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
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