linux-kernel - Re: [PATCH] x86/bugs: Allow STIBP with IBRS

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Message-ID: <Y/O9U4KFtXE8Yoyt@zn.tnic>
Date:   Mon, 20 Feb 2023 19:34:59 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     KP Singh <kpsingh@...nel.org>, linux-kernel@...r.kernel.org,
        pjt@...gle.com, evn@...gle.com, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
        hpa@...or.com, peterz@...radead.org,
        pawan.kumar.gupta@...ux.intel.com, kim.phillips@....com,
        alexandre.chartre@...cle.com, daniel.sneddon@...ux.intel.com,
        José Oliveira <joseloliveira11@...il.com>,
        Rodrigo Branco <rodrigo@...nelhacking.com>,
        Alexandra Sandulescu <aesa@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] x86/bugs: Allow STIBP with IBRS

Drop stable@ again.

On Mon, Feb 20, 2023 at 10:27:17AM -0800, Josh Poimboeuf wrote:
> IBRS is only enabled in kernel space.  Since it's not enabled in user
> space, user space isn't protected from indirect branch prediction
> attacks from a sibling CPU thread.
> 
> Allow STIBP to be enabled to protect against such attacks.
> 
> Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")

Yah, look at that one:

commit 7c693f54c873691a4b7da05c7e0f74e67745d144
Author: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Date:   Tue Jun 14 23:15:55 2022 +0200

    x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS

    Extend spectre_v2= boot option with Kernel IBRS.

    [jpoimboe: no STIBP with IBRS]
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I'm assuming this was supposed to mean no STIBP in *kernel mode* when
IBRS is selected?

In user mode, STIBP should be selectable as we disable IBRS there.

Close?

If so, pls document it too while at it:

Documentation/admin-guide/hw-vuln/spectre.rst

because we will be wondering next time again.

Like we wonder each time this madness is being touched. ;-(

Thx.

-- 
Regards/Gruss,
    Boris.

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