linux-kernel - Re: [RFC][PATCH 02/17] x86/cpu: Clean up SRSO return thunk mess

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Date:   Thu, 10 Aug 2023 14:56:31 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, jpoimboe@...nel.org,
        gregkh@...uxfoundation.org
Subject: Re: [RFC][PATCH 02/17] x86/cpu: Clean up SRSO return thunk mess

On Thu, Aug 10, 2023 at 02:37:56PM +0200, Peter Zijlstra wrote:
> It does; so zen1/2 use the decoder thing to flush BTB entry of the RET,
> both retbleed and srso do.
> 
> Then zen3/4 use the aliassing trick to flush the BTB entry of the RET.

Yes, I was correcting your "instruction aliasing". It is "BTB aliasing"
by causing those bits in the VAs to XOR.

> Then both srso options use RSB/RAP stuffing to force a mispredict there.

They cause the RETs to mispredict - no stuffing. That's the add $8,
%rsp in the zen3/4 case which causes the RET to mispredict. There's no
doing a bunch of CALLs to stuff something.

> Retbleed doesn't do this.
> 
> retbleed is about BTB, srso does both BTB and RSB/RAP.

Yes.

> So this patch doesn't actually change anything except one layer of
> indirection.

I agree with everything from here on to the end. Provided we can do that
and there's no some microarchitectural catch there, I'm all for removing
the __ret alternative.

Thx.

-- 
Regards/Gruss,
    Boris.

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