linux-kernel - Re: [RFC PATCH] x86/retpolines: Prevent speculation after RET

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Date:   Fri, 19 Feb 2021 09:15:07 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, tony.luck@...el.com, pjt@...gle.com,
        linux-kernel@...r.kernel.org, r.marek@...embler.cz,
        jpoimboe@...hat.com, jikos@...nel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [RFC PATCH] x86/retpolines: Prevent speculation after RET

On Thu, Feb 18, 2021 at 08:11:38PM +0100, Borislav Petkov wrote:
> On Thu, Feb 18, 2021 at 08:02:31PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 18, 2021 at 07:46:39PM +0100, Borislav Petkov wrote:
> > > Both vendors speculate after a near RET in some way:
> > > 
> > > Intel:
> > > 
> > > "Unlike near indirect CALL and near indirect JMP, the processor will not
> > > speculatively execute the next sequential instruction after a near RET
> > > unless that instruction is also the target of a jump or is a target in a
> > > branch predictor."
> > 
> > Right, the way I read that means it's not a problem for us here.
> 
> Look at that other thread: the instruction *after* the RET can be
> speculatively executed if that instruction is the target of a jump or it
> is in a branch predictor.

Right, but that has nothing to do with the RET instruction itself. You
can speculatively execute any random instruction by training the BTB,
which is I suppose the entire point of things :-)

So the way I read it is that: RET does not 'leak' speculation, but if
you target the instruction after RET with any other speculation crud,
ofcourse you can get it to 'run'.

And until further clarified, I'll stick with that :-)