[xen stable-4.18] x86/spec: fix INDIRECT_THUNK option to only be set when build-enabled

[patchbot@xxxxxxx]

commit 09b9db0413b1f31f27bece07b2bfa1723b89ace6
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Tue Mar 5 11:50:53 2024 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Mar 5 11:50:53 2024 +0100

    x86/spec: fix INDIRECT_THUNK option to only be set when build-enabled
    
    Attempt to provide a more helpful error message when the user attempts to 
set
    spec-ctrl=bti-thunk option but the support is build-time disabled.
    
    While there also adjust the command line documentation to mention
    CONFIG_INDIRECT_THUNK instead of INDIRECT_THUNK.
    
    Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 8441fa806a3b778867867cd0159fa1722e90397e
    master date: 2024-02-27 14:58:20 +0100
---
 docs/misc/xen-command-line.pandoc | 10 +++++-----
 xen/arch/x86/spec_ctrl.c          |  7 ++++++-
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 8e65f8bd18..582d6741d1 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2424,11 +2424,11 @@ guests to use.
   performance reasons dom0 is unprotected by default.  If it is necessary to
   protect dom0 too, boot with `spec-ctrl=ibpb-entry`.
 
-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), with the
-alternatives being `jmp` (a `jmp *%reg` gadget, minimal overhead), and
-`lfence` (an `lfence; jmp *%reg` gadget).
+If Xen was compiled with `CONFIG_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), 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 04e508b622..99ecfb3cba 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -241,7 +241,12 @@ static int __init cf_check parse_spec_ctrl(const char *s)
         {
             s += 10;
 
-            if ( !cmdline_strcmp(s, "retpoline") )
+            if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
+            {
+                no_config_param("INDIRECT_THUNK", "spec-ctrl", s - 10, ss);
+                rc = -EINVAL;
+            }
+            else if ( !cmdline_strcmp(s, "retpoline") )
                 opt_thunk = THUNK_RETPOLINE;
             else if ( !cmdline_strcmp(s, "lfence") )
                 opt_thunk = THUNK_LFENCE;
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.18