linux-kernel - RE: [PATCH 3/3] x86/bugs: Simplify SSB cmdline parsing

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Message-ID:
 <LV3PR12MB92653604B5839C580C4BA1949431A@LV3PR12MB9265.namprd12.prod.outlook.com>
Date: Mon, 18 Aug 2025 17:55:35 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
CC: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>, "H . Peter Anvin" <hpa@...or.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 3/3] x86/bugs: Simplify SSB cmdline parsing

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
> Sent: Monday, August 18, 2025 12:42 PM
> To: Kaplan, David <David.Kaplan@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>; Borislav Petkov <bp@...en8.de>; Peter
> Zijlstra <peterz@...radead.org>; Josh Poimboeuf <jpoimboe@...nel.org>; Ingo
> Molnar <mingo@...hat.com>; Dave Hansen <dave.hansen@...ux.intel.com>;
> x86@...nel.org; H . Peter Anvin <hpa@...or.com>; linux-kernel@...r.kernel.org
> Subject: Re: [PATCH 3/3] x86/bugs: Simplify SSB cmdline parsing
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Mon, Aug 11, 2025 at 09:26:59AM -0500, David Kaplan wrote:
> > Simplify the SSB command line parsing by selecting a mitigation directly,
> > as is done in most of the simpler vulnerabilities.  Use early_param instead
> > of cmdline_find_option for consistency with the other mitigation
> > selections.
> >
> > Signed-off-by: David Kaplan <david.kaplan@....com>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 118 ++++++++++++-------------------------
> >  1 file changed, 39 insertions(+), 79 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 19a3891953c3..3766dff9a699 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -2625,16 +2625,8 @@ void cpu_bugs_smt_update(void)
> >  #undef pr_fmt
> >  #define pr_fmt(fmt)  "Speculative Store Bypass: " fmt
> >
> > -static enum ssb_mitigation ssb_mode __ro_after_init =
> SPEC_STORE_BYPASS_NONE;
> > -
> > -/* The kernel command line selection */
> > -enum ssb_mitigation_cmd {
> > -     SPEC_STORE_BYPASS_CMD_NONE,
> > -     SPEC_STORE_BYPASS_CMD_AUTO,
> > -     SPEC_STORE_BYPASS_CMD_ON,
> > -     SPEC_STORE_BYPASS_CMD_PRCTL,
> > -     SPEC_STORE_BYPASS_CMD_SECCOMP,
> > -};
> > +static enum ssb_mitigation ssb_mode __ro_after_init =
> > +     IS_ENABLED(CONFIG_MITIGATION_SSB) ?
> SPEC_STORE_BYPASS_PRCTL : SPEC_STORE_BYPASS_NONE;
>
> Other mitigations default to "AUTO", but not this one. Isn't that something
> that attack-vector controls rely on?

I hadn't modified SSB in the initial attack vector series because SSB mitigation was never actually turned on (set to SPEC_STORE_BYPASS_DISABLE) by default.  The default option requires applications to opt-in to mitigation.

However this probably could be done better by saying that ssb defaults to SPEC_STORE_BYPASS_PRCTL if the user->user vector is enabled...and otherwise it is SPEC_STORE_BYPASS_NONE, since the default ssb mitigation is only for applications that want to protect themselves.

I can do that in a separate patch in this series after this clean-up patch if that sounds reasonable.

Thanks
--David Kaplan