linux-kernel - Re: [PATCH v2 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS

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Message-ID: <CACYkzJ7pLhJ+NfUq36PaMxadkgv-cPtO60TW=g_Nh7vU1vEWqA@mail.gmail.com>
Date:   Tue, 21 Feb 2023 11:35:29 -0800
From:   KP Singh <kpsingh@...nel.org>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, pjt@...gle.com, evn@...gle.com,
        jpoimboe@...nel.org, tglx@...utronix.de, 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,
        corbet@....net, bp@...e.de, linyujun809@...wei.com,
        jmattson@...gle.com,
        José Oliveira <joseloliveira11@...il.com>,
        Rodrigo Branco <rodrigo@...nelhacking.com>,
        Alexandra Sandulescu <aesa@...gle.com>, stable@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86/speculation: Allow enabling STIBP with legacy IBRS

On Tue, Feb 21, 2023 at 11:29 AM Greg KH <gregkh@...uxfoundation.org> wrote:
>
> On Tue, Feb 21, 2023 at 07:49:07PM +0100, KP Singh wrote:
> > Setting the IBRS bit implicitly enables STIBP to protect against
> > cross-thread branch target injection. With enhanced IBRS, the bit it set
> > once and is not cleared again. However, on CPUs with just legacy IBRS,
> > IBRS bit set on user -> kernel and cleared on kernel -> user (a.k.a
> > KERNEL_IBRS). Clearing this bit also disables the implicitly enabled
> > STIBP, thus requiring some form of cross-thread protection in userspace.
> >
> > Enable STIBP, either opt-in via prctl or seccomp, or always on depending
> > on the choice of mitigation selected via spectre_v2_user.
> >
> > Reported-by: José Oliveira <joseloliveira11@...il.com>
> > Reported-by: Rodrigo Branco <rodrigo@...nelhacking.com>
> > Reviewed-by: Alexandra Sandulescu <aesa@...gle.com>
> > Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS")
> > Cc: stable@...r.kernel.org
> > Signed-off-by: KP Singh <kpsingh@...nel.org>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 33 ++++++++++++++++++++++-----------
> >  1 file changed, 22 insertions(+), 11 deletions(-)
>
> Why isn't patch 2/2 for stable as well?

It should be. I actually forgot to remove stable from the first one as
there are still ongoing discussions and people kept having to "drop
stable".  I can send a v3 with stable Cc'ed. Should it have a fixes
tag too?

>
> thanks,
>
> greg k-h