linux-kernel - Re: [RFC][PATCH 00/17] Fix up the recent SRSO patches

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Message-ID: <20230809115829.GJ212435@hirez.programming.kicks-ass.net>
Date:   Wed, 9 Aug 2023 13:58:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrew.Cooper3@...rix.com
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, David.Kaplan@....com,
        jpoimboe@...nel.org, gregkh@...uxfoundation.org
Subject: Re: [RFC][PATCH 00/17] Fix up the recent SRSO patches

On Wed, Aug 09, 2023 at 11:04:15AM +0100, Andrew.Cooper3@...rix.com wrote:
> On 09/08/2023 8:12 am, Peter Zijlstra wrote:
> > Since I wasn't invited to the party (even though I did retbleed), I get to
> > clean things up afterwards :/
> >
> > Anyway, this here overhauls the SRSO patches in a big way.
> >
> > I claim that AMD retbleed (also called Speculative-Type-Confusion
> 
> Branch Type Confusion.

Durr, I shoud've double checked, and yes, too damn many different things
and not enough sleep.

> >  -- not to be
> > confused with Intel retbleed, which is an entirely different bug) is
> > fundamentally the same as this SRSO -- which is also caused by STC. And the
> > mitigations are so similar they should all be controlled from a single spot and
> > not conflated like they are now.
> 
> BTC and SRSO are certainly related, but they're not the same.
> 
> With BTC, an attacker poisons a branch type prediction to say "that
> thing (which isn't actually a ret) is a ret".
> 
> With SRSO, an attacker leaves a poisoned infinite-call-loop prediction. 
> Later, a real function (that is architecturally correct execution and
> will retire) trips over the predicted infinite loop, which overflows the
> RSB/RAS/RAP replacing the correct prediction on the top with the
> attackers choice of value.
> 
> So while branch type confusion is used to poison the top-of-RSB value,
> the ret that actually goes wrong needs a correct type=ret prediction for
> the SRSO attack to succeed.

Yes, this is what I meant, and I clearly failed to express myself
better. The point was that branch-type-confusion is involved with both,
just in different ways.