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

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Date:   Sun, 21 Jan 2018 20:07:06 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
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 20:54 +0100, Borislav Petkov wrote:
> On Sun, Jan 21, 2018 at 07:31:39PM +0000, David Woodhouse wrote:
> > >         if (boot_cpu_has(X86_FEATURE_IBPB))
> > >                 wrmsr(MSR_IA32_PRED_CMD, PRED_CMD_IBPB, 0);
> > > 
> > > Problem solved.
> > 
> > Nope. Plenty of patch sets *did* have the bug above though, until it
> > was spotted.
> 
> And that bug is...?

That bug is the *reason* we're arguing about static_cpu_has vs.
ALTERNATIVE. 

A conditional branch that the CPU sees can be speculated over...

Now, Andrew is right that in a number of cases there will be another
serialising instruction before we ever hit a problematic indirect
branch. But as I just said elsewhere, I'd really like the *primitives*
to support unconditional operation.
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