linux-kernel - Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

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Message-ID: <1516563060.9814.52.camel@infradead.org>
Date:   Sun, 21 Jan 2018 19:31:00 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>,
        KarimAllah Ahmed <karahmed@...zon.com>
Cc:     arjan@...ux.intel.com, tglx@...utronix.de, karahmed@...zon.de,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        tim.c.chen@...ux.intel.com, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org
Subject: Re: [PATCH v2 5/8] x86/speculation: Add basic support for IBPB

On Sun, 2018-01-21 at 20:01 +0100, Borislav Petkov wrote:
> 
> so execution runs directly into the MSR write and the JMP is gone.
> 
> So I don't see indirect branches anywhere...

Wait until the wind changes.

Congratulations, you've just turned a potential GCC missed optimisation
into a kernel bug. We don't *care* that it's unlikely that GCC will
miss that optimisation. The point is that it doesn't *have* to do it,
and we don't *check*.

cf. https://lkml.org/lkml/2018/1/12/176

... after which Peter went off and implemented that check, which is all
fine and dandy but let's not rely on backporting that too.
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