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

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Message-ID: <20210218191138.GH4214@zn.tnic>
Date:   Thu, 18 Feb 2021 20:11:38 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
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 08:02:31PM +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.

And yes, the text is confusing and no one from Intel has clarified
definitively yet what that text means exactly.

> Now, if AMD were to say something like: hey, that retpoline is pretty
> awesome, we ought to use that instead of an uconditional LFENCE, then
> sure, but as is, I don't think so.

AMD prefers the LFENCE instead of the ratpoline sequence.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette