linux-kernel - Re: [PATCH v3] x86/bugs: Enable STIBP for IBPB mitigated RetBleed

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Date:   Mon, 8 Aug 2022 16:23:04 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Kim Phillips <kim.phillips@....com>
Cc:     mingo@...nel.org, andrew.cooper3@...rix.com, bp@...en8.de,
        bp@...e.de, bpf@...r.kernel.org, jpoimboe@...hat.com,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        thomas.lendacky@....com, x86@...nel.org
Subject: Re: [PATCH v3] x86/bugs: Enable STIBP for IBPB mitigated RetBleed

On Mon, Aug 08, 2022 at 09:17:02AM -0500, Kim Phillips wrote:
> AMD's "Technical Guidance for Mitigating Branch Type Confusion,
> Rev. 1.0 2022-07-12" whitepaper, under section 6.1.2 "IBPB On
> Privileged Mode Entry / SMT Safety" says:
> 
> "Similar to the Jmp2Ret mitigation, if the code on the sibling thread
> cannot be trusted, software should set STIBP to 1 or disable SMT to
> ensure SMT safety when using this mitigation."
> 
> So, like already being done for retbleed=unret, the also for
> retbleed=ibpb, force STIBP on machines that have it, and report
> its SMT vulnerability status accordingly.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=206537
> Fixes: 3ebc17006888 ("x86/bugs: Add retbleed=ibpb")
> Signed-off-by: Kim Phillips <kim.phillips@....com>
> ---
> v3:  "unret and ibpb mitigations" -> "UNRET and IBPB mitigations" (Mingo)
> v2:  Justify and explain STIBP's role with IBPB (Boris)
> 
>  .../admin-guide/kernel-parameters.txt         | 20 ++++++++++++++-----
>  arch/x86/kernel/cpu/bugs.c                    | 10 ++++++----
>  2 files changed, 21 insertions(+), 9 deletions(-)

Any specific reason you don't want this also backported to the stable
kernel branches that have the other retbleed fixes in them?

thanks,

greg k-h