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

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Date:   Thu, 18 Feb 2021 20:02:31 +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 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.

> AMD:
> 
> "Some AMD processors when they first encounter a branch do not stall
> dispatch and use the branches dynamic execution to determine the target.
> Therefore, they will speculatively dispatch the sequential instructions
> after the branch. This happens for near return instructions where it is
> not clear what code may exist sequentially after the return instruction.
> This behavior also occurs with jmp/call instructions with indirect
> targets. Software should place a LFENCE or another dispatch serializing
> instruction after the return or jmp/call indirect instruction to prevent
> this sequential speculation."
> 
> The AMD side doesn't really need the LFENCE because it'll do LFENCE;
> JMP/CALL <target> due to X86_FEATURE_RETPOLINE_AMD, before it reaches
> the RET.

It never reached the RET.

So all in all, I really don't see why we'd need this.

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.