Date:   Mon, 20 Feb 2023 11:09:08 -0800
From:   Josh Poimboeuf <jpoimboe@kernel.org>
To:     Borislav Petkov <bp@alien8.de>
Cc:     KP Singh <kpsingh@kernel.org>, 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?B?Sm9zw6k=?= Oliveira <joseloliveira11@gmail.com>,
        Rodrigo Branco <rodrigo@kernelhacking.com>,
        Alexandra Sandulescu <aesa@google.com>,
        Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH] x86/bugs: Allow STIBP with IBRS
Message-ID: <20230220190908.gfd2c4tuzeb7cft6@treble>
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 <20230220163442.7fmaeef3oqci4ee3@treble>
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On Mon, Feb 20, 2023 at 07:34:59PM +0100, Borislav Petkov wrote:
> 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@linux.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?

No it was supposed to be "no STIBP with *eIBRS*".

> 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. ;-(

As far as I can tell, that document was never updated to describe
spectre_v2=ibrs in the first place.  That would be a whole 'nother patch
which I'm not volunteering for.  Nice try ;-)

-- 
Josh