linux-kernel - RE: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID:
 <DS0PR12MB92731E9877D21E36EC3EE4ED94E9A@DS0PR12MB9273.namprd12.prod.outlook.com>
Date: Thu, 16 Oct 2025 15:34:14 +0000
From: "Kaplan, David" <David.Kaplan@....com>
To: Peter Zijlstra <peterz@...radead.org>
CC: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, Josh
 Poimboeuf <jpoimboe@...nel.org>, Pawan Gupta
	<pawan.kumar.gupta@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, Dave
 Hansen <dave.hansen@...ux.intel.com>, "x86@...nel.org" <x86@...nel.org>, "H .
 Peter Anvin" <hpa@...or.com>, Alexander Graf <graf@...zon.com>, Boris
 Ostrovsky <boris.ostrovsky@...cle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi_safe()

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Peter Zijlstra <peterz@...radead.org>
> Sent: Thursday, October 16, 2025 9:48 AM
> To: Kaplan, David <David.Kaplan@....com>
> Cc: Thomas Gleixner <tglx@...utronix.de>; Borislav Petkov <bp@...en8.de>; Josh
> Poimboeuf <jpoimboe@...nel.org>; Pawan Gupta
> <pawan.kumar.gupta@...ux.intel.com>; Ingo Molnar <mingo@...hat.com>; Dave
> Hansen <dave.hansen@...ux.intel.com>; x86@...nel.org; H . Peter Anvin
> <hpa@...or.com>; Alexander Graf <graf@...zon.com>; Boris Ostrovsky
> <boris.ostrovsky@...cle.com>; linux-kernel@...r.kernel.org
> Subject: Re: [RFC PATCH 40/56] x86/alternative: Use sync_core_nmi_safe()
>
> Caution: This message originated from an External Source. Use proper caution
> when opening attachments, clicking links, or responding.
>
>
> On Thu, Oct 16, 2025 at 02:40:51PM +0000, Kaplan, David wrote:
>
> > > Can we please keep this in sync_core()? Something like:
> > >
> > > static __always_inline void sync_core(void)
> > > {
> > >         if (static_cpu_has(X86_FEATURE_SERIALIZE)) {
> > >                 serialize();
> > >                 return;
> > >         }
> > >
> > > +       if (repatch_in_progress) {
> > > +               sync_core_nmi_safe();
> > > +               return;
> > > +       }
> > > +
> > >         iret_to_self();
> > > }
> > >
> > > That way all the modern stuff that has SERIALIZE will still use that.
> >
> > Hmm, I can't quite do that because sync_core() is used in a number of
> > other places too (unless we make repatch_in_progress a true global).
>
> We could just out-of-line the thing; nothing using this should care
> about cycles -- all of this is quite expensive.

I guess I could also just add the SERIALIZE check into sync_core_nmi_safe() and prefer that over mov-cr2.  Although as you said, cycles shouldn't matter...

>
> > I wonder though if it'd be ok to have sync_core() check
> > IS_ENABLED(CONFIG_DYNAMIC_MITIGATIONS) and then always use the
> mov-cr2
> > version?  It might also have to check X86_FEATURE_XENPV and use IRET
> > in that case but otherwise I'd think it's safe for machines that could
> > support dynamic mitigations.
>
> Yeah, dunno.. I'm not well versed in the virt thing.
>
> BTW, will AMD do that SERIALIZE instruction?

It is not supported on current parts, I can't comment on future ones.

I will note that on AMD, MFENCE has the required serializing properties (like SERIALIZE).  On AMD, we could use MFENCE in sync_core() which is probably faster that iret_to_self().  But again...do we really care.

--David Kaplan