$NetBSD: patch-XSA398,v 1.1 2022/06/24 13:47:37 bouyer Exp $

From 7b9814b250a5a28277bd0866d341a5cfc0f4c1ac Mon Sep 17 00:00:00 2001
From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Mon, 7 Mar 2022 16:35:52 +0000
Subject: x86/spec-ctrl: Cease using thunk=lfence on AMD

AMD have updated their Spectre v2 guidance, and lfence/jmp is no longer
considered safe.  AMD are recommending using retpoline everywhere.

Update the default heuristics to never select THUNK_LFENCE.

This is part of XSA-398 / CVE-2021-26401.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 8d03080d2a339840d3a59e0932a94f804e45110d)

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index cf9dea62dbfd..eead69ada2c2 100644
--- docs/misc/xen-command-line.pandoc.orig
+++ docs/misc/xen-command-line.pandoc
@@ -2077,9 +2077,9 @@ to use.
 
 If Xen was compiled with INDIRECT_THUNK support, `bti-thunk=` can be used to
 select which of the thunks gets patched into the `__x86_indirect_thunk_%reg`
-locations.  The default thunk is `retpoline` (generally preferred for Intel
-hardware), with the alternatives being `jmp` (a `jmp *%reg` gadget, minimal
-overhead), and `lfence` (an `lfence; jmp *%reg` gadget, preferred for AMD).
+locations.  The default thunk is `retpoline` (generally preferred), with the
+alternatives being `jmp` (a `jmp *%reg` gadget, minimal overhead), and
+`lfence` (an `lfence; jmp *%reg` gadget).
 
 On hardware supporting IBRS (Indirect Branch Restricted Speculation), the
 `ibrs=` option can be used to force or prevent Xen using the feature itself.
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 1cfd02d7d7cf..7447d4a8e5b5 100644
--- xen/arch/x86/spec_ctrl.c.orig
+++ xen/arch/x86/spec_ctrl.c
@@ -908,16 +908,10 @@ void __init init_speculation_mitigations(void)
         if ( IS_ENABLED(CONFIG_INDIRECT_THUNK) )
         {
             /*
-             * AMD's recommended mitigation is to set lfence as being dispatch
-             * serialising, and to use IND_THUNK_LFENCE.
-             */
-            if ( cpu_has_lfence_dispatch )
-                thunk = THUNK_LFENCE;
-            /*
-             * On Intel hardware, we'd like to use retpoline in preference to
+             * On all hardware, we'd like to use retpoline in preference to
              * IBRS, but only if it is safe on this hardware.
              */
-            else if ( retpoline_safe(caps) )
+            if ( retpoline_safe(caps) )
                 thunk = THUNK_RETPOLINE;
             else if ( boot_cpu_has(X86_FEATURE_IBRSB) )
                 ibrs = true;