[xen stable-4.16] x86/spec-ctrl: Cease using thunk=lfence on AMD

[patchbot@xxxxxxx]

commit c374a8c5cc74535e16410b7a0d9e92bf5de54f79
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Mar 7 16:35:52 2022 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Mar 8 17:15:41 2022 +0000

    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.
    
    Retpoline is incompatible with CET.  All CET-capable hardware has efficient
    IBRS (specifically, not something retrofitted in microcode), so use IBRS 
(and
    STIBP for consistency sake).
    
    This is a logical change on AMD, but not on Intel as the default 
calculations
    would end up with these settings anyway.  Leave behind a message if IBRS is
    found to be missing.
    
    Also update the default heuristics to never select THUNK_LFENCE.  This 
causes
    AMD CPUs to change their default to retpoline.
    
    Also update the printed message to include the AMD MSR_SPEC_CTRL settings, 
and
    STIBP now that we set it for consistency sake.
    
    This is part of XSA-398 / CVE-2021-26401.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 8d03080d2a339840d3a59e0932a94f804e45110d)
---
 docs/misc/xen-command-line.pandoc |  6 +++---
 xen/arch/x86/spec_ctrl.c          | 40 +++++++++++++++++++++++++--------------
 2 files changed, 29 insertions(+), 17 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 995197f4b2..f606dc0e14 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2269,9 +2269,9 @@ 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 cbeeb19903..ae076bec3a 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -367,14 +367,19 @@ static void __init print_details(enum ind_thunk thunk, 
uint64_t caps)
                "\n");
 
     /* Settings for Xen's protection, irrespective of guests. */
-    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s, 
Other:%s%s%s%s%s\n",
+    printk("  Xen settings: BTI-Thunk %s, SPEC_CTRL: %s%s%s%s, 
Other:%s%s%s%s%s\n",
            thunk == THUNK_NONE      ? "N/A" :
            thunk == THUNK_RETPOLINE ? "RETPOLINE" :
            thunk == THUNK_LFENCE    ? "LFENCE" :
            thunk == THUNK_JMP       ? "JMP" : "?",
-           !boot_cpu_has(X86_FEATURE_IBRSB)          ? "No" :
+           (!boot_cpu_has(X86_FEATURE_IBRSB) &&
+            !boot_cpu_has(X86_FEATURE_IBRS))         ? "No" :
            (default_xen_spec_ctrl & SPEC_CTRL_IBRS)  ? "IBRS+" :  "IBRS-",
-           !boot_cpu_has(X86_FEATURE_SSBD)           ? "" :
+           (!boot_cpu_has(X86_FEATURE_STIBP) &&
+            !boot_cpu_has(X86_FEATURE_AMD_STIBP))    ? "" :
+           (default_xen_spec_ctrl & SPEC_CTRL_STIBP) ? " STIBP+" : " STIBP-",
+           (!boot_cpu_has(X86_FEATURE_SSBD) &&
+            !boot_cpu_has(X86_FEATURE_AMD_SSBD))     ? "" :
            (default_xen_spec_ctrl & SPEC_CTRL_SSBD)  ? " SSBD+" : " SSBD-",
            !(caps & ARCH_CAPS_TSX_CTRL)              ? "" :
            (opt_tsx & 1)                             ? " TSX+" : " TSX-",
@@ -945,10 +950,23 @@ void __init init_speculation_mitigations(void)
     /*
      * First, disable the use of retpolines if Xen is using shadow stacks, as
      * they are incompatible.
+     *
+     * 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 &&
-         (opt_thunk == THUNK_DEFAULT || opt_thunk == THUNK_RETPOLINE) )
-        thunk = THUNK_JMP;
+    if ( cpu_has_xen_shstk )
+    {
+        if ( !has_spec_ctrl )
+            printk(XENLOG_WARNING "?!? CET active, but no MSR_SPEC_CTRL?\n");
+        else if ( opt_ibrs == -1 )
+        {
+            opt_ibrs = ibrs = true;
+            default_xen_spec_ctrl |= SPEC_CTRL_IBRS | SPEC_CTRL_STIBP;
+        }
+
+        if ( opt_thunk == THUNK_DEFAULT || opt_thunk == THUNK_RETPOLINE )
+            thunk = THUNK_JMP;
+    }
 
     /*
      * Has the user specified any custom BTI mitigations?  If so, follow their
@@ -968,16 +986,10 @@ 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 ( has_spec_ctrl )
                 ibrs = true;
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.16