linux-kernel - Re: [PATCH 04/22] x86/srso: Fix SBPB enablement for spec_rstack

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Date:   Wed, 23 Aug 2023 03:27:28 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Babu Moger <babu.moger@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, David.Kaplan@....com,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Nikolay Borisov <nik.borisov@...e.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 04/22] x86/srso: Fix SBPB enablement for
 spec_rstack_overflow=off

On Tue, Aug 22, 2023 at 02:59:01PM -0700, Josh Poimboeuf wrote:
> Yeah, I had seen that.  The combination of spectre_v2_user=on with
> srso=off doesn't make a whole lot of sense, but... give the user what
> they want and all.  Which would presumably be IBPB *without* the SRSO
> mitigation (aka SBPB).

Right.

> I don't think we need to worry about that, SBPB is >= fam19 but retbleed
> is <= fam17.  So either way (0x17 or 0x19) entry_ibpb() should do IBPB.

Right, and SBPB is possible only on family => 0x19 anyway which is not
affected by retbleed.

I was worried that we might open some mitigation hole with the overwrite
but it seems we're fine:

family 0x17: retbleed=ibpb, srso=off - is fine, can't do SBPB anyway
family 0x19: retbleed=ibpb, srso=off - fine too, not affected by retbleed, can do SBPB

Unless I'm missing something again which is very likely, by now. ;-\

Thx.

-- 
Regards/Gruss,
    Boris.

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