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

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Message-ID: <20230814200128.ioas5lk2r3yzfkkv@treble>
Date:   Mon, 14 Aug 2023 13:01:28 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, gregkh@...uxfoundation.org,
        nik.borisov@...e.com
Subject: Re: [PATCH v2 00/11] Fix up SRSO stuff

On Mon, Aug 14, 2023 at 12:51:55PM -0700, Josh Poimboeuf wrote:
> On Mon, Aug 14, 2023 at 06:44:47PM +0200, Borislav Petkov wrote:
> > 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.
> >  */
> 
> It's a good comment, but RET in srso_safe_ret() is still misaligned.
> Don't we need something like so?

Scratch that, I guess I misread the confusingly worded comment:

  "both the ret in zen_untrain_ret() and srso_safe_ret()..."

to mean the RET in each function.

How about:

  "both the RET in zen_untrain_ret() and the LEA in srso_untrain_ret()"

?

-- 
Josh