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

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Message-ID: <20230810115148.GEZNTPVLBmPL6uz4Af@fat_crate.local>
Date:   Thu, 10 Aug 2023 13:51:48 +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 Wed, Aug 09, 2023 at 09:12:20AM +0200, Peter Zijlstra wrote:
> Where Zen1/2 flush the BTB using the instruction decoder trick
> (test,movabs) Zen3/4 use instruction aliasing. SRSO adds RSB (RAP in

BTB aliasing.

> AMD speak) stuffing to force a return mis-predict.

No it doesn't. It causes BTB aliasing which evicts any potentially
poisoned entries.

> That is; the AMD retbleed is a form of Speculative-Type-Confusion
> where the branch predictor is trained to use the BTB to predict the
> RET address, while AMD inception/SRSO is a form of
> Speculative-Type-Confusion where another instruction is trained to be
> treated like a CALL instruction and poison the RSB (RAP).

Nope, Andy explained it already in the 0th message.

> Pick one of three options at boot.

Yes, provided microarchitecturally that works, I'm all for removing the
__ret alternative.

Thx.

-- 
Regards/Gruss,
    Boris.

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