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@citrix.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Changes since v2:
- Adjust documentation.
- Use IS_ENABLED() instead of #ifdef.
Changes since v1:
- New in this version.
---
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 be76be8d5365..02896598df6f 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2417,11 +2417,11 @@ 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 5fae80774519..ca82b9e41ccd 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=bti-thunk", s, ss);
+ rc = -EINVAL;
+ }
+ else if ( !cmdline_strcmp(s, "retpoline") )
opt_thunk = THUNK_RETPOLINE;
else if ( !cmdline_strcmp(s, "lfence") )
opt_thunk = THUNK_LFENCE;
--
2.43.0
On 26.02.2024 12:07, Roger Pau Monne wrote:
> 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@citrix.com>
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
with one minor remark:
> --- 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=bti-thunk", s, ss);
> + rc = -EINVAL;
> + }
> + else if ( !cmdline_strcmp(s, "retpoline") )
> opt_thunk = THUNK_RETPOLINE;
> else if ( !cmdline_strcmp(s, "lfence") )
> opt_thunk = THUNK_LFENCE;
How about
if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
{
no_config_param("INDIRECT_THUNK", "spec-ctrl", s - 10, ss);
rc = -EINVAL;
}
else if ( !cmdline_strcmp(s, "retpoline") )
or (likely less liked by you and Andrew) "s += 10;" dropped and then
if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
{
no_config_param("INDIRECT_THUNK", "spec-ctrl", s, ss);
rc = -EINVAL;
}
else if ( !cmdline_strcmp(s += 10, "retpoline") )
conserving a little on string literal space (sadly, despite the function
being __init, string literals remain post-init due to living in .rodata)?
Jan
On Mon, Feb 26, 2024 at 01:50:46PM +0100, Jan Beulich wrote:
> On 26.02.2024 12:07, Roger Pau Monne wrote:
> > 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@citrix.com>
> > Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
>
> Reviewed-by: Jan Beulich <jbeulich@suse.com>
> with one minor remark:
>
> > --- 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=bti-thunk", s, ss);
> > + rc = -EINVAL;
> > + }
> > + else if ( !cmdline_strcmp(s, "retpoline") )
> > opt_thunk = THUNK_RETPOLINE;
> > else if ( !cmdline_strcmp(s, "lfence") )
> > opt_thunk = THUNK_LFENCE;
>
> How about
>
> if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
> {
> no_config_param("INDIRECT_THUNK", "spec-ctrl", s - 10, ss);
> rc = -EINVAL;
> }
> else if ( !cmdline_strcmp(s, "retpoline") )
>
> or (likely less liked by you and Andrew) "s += 10;" dropped and then
>
> if ( !IS_ENABLED(CONFIG_INDIRECT_THUNK) )
> {
> no_config_param("INDIRECT_THUNK", "spec-ctrl", s, ss);
> rc = -EINVAL;
> }
> else if ( !cmdline_strcmp(s += 10, "retpoline") )
>
> conserving a little on string literal space (sadly, despite the function
> being __init, string literals remain post-init due to living in .rodata)?
TBH I'm not overly worried about those 10 extra characters, but if I
had to choose I prefer the first option (so `s - 10`).
Thanks, Roger.
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