Re: [PATCH v3] x86/bugs: Only harden syscalls when needed

[Josh Poimboeuf <jpoimboe@kernel.org>]

On Wed, Apr 17, 2024 at 07:01:54PM +0100, Andrew Cooper wrote:
> On 17/04/2024 6:57 pm, Josh Poimboeuf wrote:
> > On Wed, Apr 17, 2024 at 09:45:14AM -0700, Pawan Gupta wrote:
> >> On Wed, Apr 17, 2024 at 04:14:26PM +0100, Andrew Cooper wrote:
> >>> On 17/04/2024 12:02 am, Josh Poimboeuf wrote:
> >>>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> >>>> index ca295b0c1eee..dcb97cc2758f 100644
> >>>> --- a/arch/x86/kernel/cpu/bugs.c
> >>>> +++ b/arch/x86/kernel/cpu/bugs.c
> >>>> @@ -1678,6 +1687,21 @@ static void __init spectre_v2_select_mitigation(void)
> >>>>  	enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline();
> >>>>  	enum spectre_v2_mitigation mode = SPECTRE_V2_NONE;
> >>>>  
> >>>> +	/*
> >>>> +	 * X86_FEATURE_INDIRECT_SAFE indicates whether indirect calls can be
> >>>> +	 * considered safe.  That means either:
> >>>> +	 *
> >>>> +	 *   - the CPU isn't vulnerable to Spectre v2 or its variants;
> >>>> +	 *
> >>>> +	 *   - a hardware mitigation is in place (e.g., IBRS, BHI_DIS_S); or
> >>>> +	 *
> >>>> +	 *   - the user turned off mitigations altogether.
> >>>> +	 *
> >>>> +	 * Assume innocence until proven guilty: set the cap bit now, then
> >>>> +	 * clear it later if/when needed.
> >>>> +	 */
> >>>> +	setup_force_cpu_cap(X86_FEATURE_INDIRECT_SAFE);
> >>> Following on from the (re)discovery that X86_FEATURE_RETPOLINE is a poor
> >>> name given what it *actually* does, can I recommend s/SAFE/OK/ here?
> >> Or simply X86_FEATURE_USE_INDIRECT_BRANCH.
> >>
> >>> This flag really is "do I want indirect branches or not", which - as
> >>> noted here - is more than just a judgement of whether indirect branches
> >>> are "safe".
> > X86_FEATURE_USE_INDIRECT_BRANCH sounds good.  It's a bit long but does
> > describe it better.
> 
> Works for me.  Definitely an improvement over SAFE.

USE_INDIRECT_BRANCH is now irking me: "use indirect branch for what?
when? why?"

At the moment I'm leaning towards Andrew's suggestion of
X86_FEATURE_INDIRECT_OK as it at least says it's "ok" (safe or don't
care) to use indirect branches when desired (typically performance
raisins).  I'll probably go with that (or maybe
X86_FEATURE_INDIRECT_BRANCH_OK) unless anybody yells.

-- 
Josh