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From:   KP Singh <kpsingh@kernel.org>
Date:   Mon, 20 Feb 2023 10:01:57 -0800
Message-ID: <CACYkzJ71xqzY6-wL+YShcL+d6ugzcdFHr6tbYWWE_ep52+RBZQ@mail.gmail.com>
Subject: Re: [PATCH RESEND] x86/speculation: Fix user-mode spectre-v2
 protection with KERNEL_IBRS
To:     Josh Poimboeuf <jpoimboe@kernel.org>
Cc:     Borislav Petkov <bp@alien8.de>, linux-kernel@vger.kernel.org,
        pjt@google.com, evn@google.com, tglx@linutronix.de,
        mingo@redhat.com, dave.hansen@linux.intel.com, x86@kernel.org,
        hpa@zytor.com, peterz@infradead.org,
        pawan.kumar.gupta@linux.intel.com, kim.phillips@amd.com,
        alexandre.chartre@oracle.com, daniel.sneddon@linux.intel.com,
        =?UTF-8?Q?Jos=C3=A9_Oliveira?= <joseloliveira11@gmail.com>,
        Rodrigo Branco <rodrigo@kernelhacking.com>,
        Alexandra Sandulescu <aesa@google.com>,
        Jim Mattson <jmattson@google.com>, stable@vger.kernel.org
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On Mon, Feb 20, 2023 at 9:59 AM Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> On Mon, Feb 20, 2023 at 06:46:04PM +0100, Borislav Petkov wrote:
> > On Mon, Feb 20, 2023 at 08:34:42AM -0800, Josh Poimboeuf wrote:
> > > We will never enable IBRS in user space.  We tried that years ago and it
> > > was very slow.
> >
> > Then I don't know what this is trying to fix.
> >
> > We'd need a proper bug statement in the commit message what the problem
> > is. As folks have established, the hw vuln mess is a whole universe of
> > crazy. So without proper documenting, this spaghetti madness will be
> > completely unreadable.
>
> Agreed, and there seems to be a lot of confusion around this patch.  I
> think I understand the issue, let me write up a new patch which is
> hopefully clearer.

Feel free to write a patch, but I don't get the confusion.

Well, we disable IBRS userspace (this is KENREL_IBRS), because it is
slow. Now if a user space process wants to protect itself from cross
thread training, it should be able to do it, either by turning STIBP
always on or using a prctl to enable. With the current logic, it's
unable to do so. All of this is mentioned in the commit message.

>
> --
> Josh