linux-kernel - Re: [PATCH v2 00/11] Fix up SRSO stuff

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Message-ID: <20230814164447.GFZNpZ/64H4lENIe94@fat_crate.local>
Date:   Mon, 14 Aug 2023 18:44:47 +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, nik.borisov@...e.com
Subject: Re: [PATCH v2 00/11] Fix up SRSO stuff

On Mon, Aug 14, 2023 at 01:44:26PM +0200, Peter Zijlstra wrote:
> The one open techinical issue I have with the mitigation is the alignment of
> the RET inside srso_safe_ret(). The details given for retbleed stated that RET
> should be on a 64byte boundary, which is not the case here.

I have written this in the hope to make this more clear:

/*
 * Some generic notes on the untraining sequences:
 *
 * They are interchangeable when it comes to flushing potentially wrong
 * RET predictions from the BTB.
 *
 * The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the
 * Retbleed sequence because the return sequence done there
 * (srso_safe_ret()) is longer and the return sequence must fully nest
 * (end before) the untraining sequence. Therefore, the untraining
 * sequence must overlap the return sequence.
 *
 * Regarding alignment - the instructions which need to be untrained,
 * must all start at a cacheline boundary for Zen1/2 generations. That
 * is, both the ret in zen_untrain_ret() and srso_safe_ret() in the
 * srso_untrain_ret() must both be placed at the beginning of
 * a cacheline.
 */

-- 
Regards/Gruss,
    Boris.

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