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

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Date:   Tue, 22 Aug 2023 08:07:06 +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 07:54:52AM +0200, Borislav Petkov wrote:
> If you goto pred_cmd, you will overwrite it with PRED_CMD_SBPB here.

Looking at this more:

"If SRSO mitigation is not required or is disabled, software may use
SBPB on context/virtual machine switch to help protect against
vulnerabilities like Spectre v2."

I think we actually want this overwrite to happen.

But then if retbleed=ibpb, entry_ibpb() will do bit 0 unconditionally...

Hmm, lemme talk to people.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette