Re: [PATCH] xen/Kconfig: Improve help test for speculative options

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 09/05/2025 9:14 am, Roger Pau Monné wrote:
> On Thu, May 08, 2025 at 05:03:36PM +0100, Andrew Cooper wrote:
>> The text for CONFIG_INDIRECT_THUNK isn't really correct, and was already 
>> stale
>> by the time speculative vulnerabilities hit the headlines in 2018.  It is
>> specifically an out-of-line-ing mechansim, and repoline is one of several
>> safety sequences used.
>>
>> Some of this boilerplate has been copied into all other options, and isn't
>> interesting for the target audience given that they're all in a "Speculative
>> Hardning" menu.
>>
>> Reword it to be more concise.
>>
>> No functional change.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>
> You are the expert on those things :).
>
>> ---
>> CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
>> CC: Michal Orzel <michal.orzel@xxxxxxx>
>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>> CC: Julien Grall <julien@xxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>>
>> CONFIG_SPECULATIVE_HARDEN_BRANCH really ought to be named
>> CONFIG_SPECULATIVE_HARDEN_CONDITIONAL, but this would be a (minor) functional
>> change.
> I don't have a strong opinion either way TBH.  Would you maybe like to
> rename the menu visible text to "Speculative Conditional Branch Hardening"?

Hmm yeah, that's better than nothing.

>
>> ---
>>  xen/common/Kconfig | 51 +++++++++-------------------------------------
>>  1 file changed, 10 insertions(+), 41 deletions(-)
>>
>> diff --git a/xen/common/Kconfig b/xen/common/Kconfig
>> index 4bec78c6f267..03ef6d87abc0 100644
>> --- a/xen/common/Kconfig
>> +++ b/xen/common/Kconfig
>> @@ -162,29 +162,21 @@ config STATIC_MEMORY
>>  menu "Speculative hardening"
>>  
>>  config INDIRECT_THUNK
>> -    bool "Speculative Branch Target Injection Protection"
>> +    bool "Out-of-line Indirect Call/Jumps"
>>      depends on CC_HAS_INDIRECT_THUNK
>>      default y
>>      help
>> -      Contemporary processors may use speculative execution as a
>> -      performance optimisation, but this can potentially be abused by an
>> -      attacker to leak data via speculative sidechannels.
> It would be nice if this boilerplate text could be made the "help" of
> the top level menu entry, but that's not possible with Kconfig.

When speculation was entirely new, something needed to introduce it (not
that I think this was great to start with), but nowadays any all
developers/sysadmins/distro-packagers will be aware of it.

Or, if they're not aware, a paragraph like this isn't going to help them.

~Andrew