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

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Message-ID: <20230822055452.GDZORNrNdYc3OmGygU@fat_crate.local>
Date:   Tue, 22 Aug 2023 07:54:52 +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 Mon, Aug 21, 2023 at 09:36:49AM -0700, Josh Poimboeuf wrote:
> Hm?  How exactly is this overriding the retbleed IBPB mitigation?

Sorry, clearly -ETOOMANYMITIGATIONS.

I meant the spectre_v2_user thing which does
indirect_branch_prediction_barrier() based on X86_FEATURE_USE_IBPB.

indirect_branch_prediction_barrier() uses x86_pred_cmd to select which
MSR bits to set and it is initialized by default to PRED_CMD_IBPB.

If you goto pred_cmd, you will overwrite it with PRED_CMD_SBPB here.

I think it should not overwrite it and simply return like before.
Meaning: if SRSO mitigation is off but the spectre_v2_user isn't so you
get what you want.

If you do mitigations=off - which is what most use cases do when they
don't care about mitigations - then it'll work too.

I don't see a sensible use case where user->user spectre_v2 is enabled
but SRSO is off. Maybe there is...

-- 
Regards/Gruss,
    Boris.

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