RE: [RFC PATCH 13/34] x86/bugs: Restructure bhi mitigation

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Nikolay Borisov <nik.borisov@xxxxxxxx>
> Sent: Tuesday, October 8, 2024 7:42 AM
> To: Kaplan, David <David.Kaplan@xxxxxxx>; Thomas Gleixner
> <tglx@xxxxxxxxxxxxx>; Borislav Petkov <bp@xxxxxxxxx>; Peter Zijlstra
> <peterz@xxxxxxxxxxxxx>; Josh Poimboeuf <jpoimboe@xxxxxxxxxx>; Pawan Gupta
> <pawan.kumar.gupta@xxxxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; Dave
> Hansen <dave.hansen@xxxxxxxxxxxxxxx>; x86@xxxxxxxxxx; H . Peter Anvin
> <hpa@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFC PATCH 13/34] x86/bugs: Restructure bhi mitigation
>
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>
> On 12.09.24 г. 22:08 ч., David Kaplan wrote:
> > Restructure bhi mitigation to use select/apply functions to create
> > consistent vulnerability handling.
> >
> > Define new AUTO mitigation for bhi.
> >
> > Signed-off-by: David Kaplan <david.kaplan@xxxxxxx>
> > ---
> >   arch/x86/kernel/cpu/bugs.c | 22 ++++++++++++++++++----
> >   1 file changed, 18 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index eaef5a1cb4a3..da6ca2fc939d 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -82,6 +82,8 @@ static void __init l1d_flush_select_mitigation(void);
> >   static void __init srso_select_mitigation(void);
> >   static void __init gds_select_mitigation(void);
> >   static void __init gds_apply_mitigation(void);
> > +static void __init bhi_select_mitigation(void); static void __init
> > +bhi_apply_mitigation(void);
> >
> >   /* The base value of the SPEC_CTRL MSR without task-specific bits set */
> >   u64 x86_spec_ctrl_base;
> > @@ -201,6 +203,7 @@ void __init cpu_select_mitigations(void)
> >        */
> >       srso_select_mitigation();
> >       gds_select_mitigation();
> > +     bhi_select_mitigation();
> >
> >       /*
> >        * After mitigations are selected, some may need to update their
> > @@ -222,6 +225,7 @@ void __init cpu_select_mitigations(void)
> >       rfds_apply_mitigation();
> >       srbds_apply_mitigation();
> >       gds_apply_mitigation();
> > +     bhi_apply_mitigation();
> >   }
> >
> >   /*
> > @@ -1719,12 +1723,13 @@ static bool __init spec_ctrl_bhi_dis(void)
> >
> >   enum bhi_mitigations {
> >       BHI_MITIGATION_OFF,
> > +     BHI_MITIGATION_AUTO,
> >       BHI_MITIGATION_ON,
> >       BHI_MITIGATION_VMEXIT_ONLY,
> >   };
>
>
> Since this series refactors all mitigations how about taking ON to mean AUTO
> which would result in overall less states for the various mitigations. If we take BHI
> as an example I don't see what value does _AUTO bring here.

In this (and the other bugs), AUTO means that no bug-specific command line option was provided.  In this way we can differentiate between no option provided (in which case attack vector controls will decide if mitigation is needed) or "bhi=on" which will force the bhi mitigation on even if the attack vector controls would otherwise leave it disabled.

--David Kaplan