linux-kernel - RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation

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Date:   Sun, 17 Jul 2022 21:15:44 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     David Laight <David.Laight@...LAB.COM>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: RE: [patch 00/38] x86/retbleed: Call depth tracking mitigation

On Sun, Jul 17 2022 at 17:56, David Laight wrote:
> From: Thomas Gleixner
>> On Sun, Jul 17 2022 at 09:45, David Laight wrote:
> I was thinking about what happens after the RSB has underflowed.
> Which is when (I presume) the BTB based speculation happens.
>
>> The intra function call in the retpoline is of course adding a RSB entry
>> which points to the speculation trap, but that gets popped immediately
>> after that by the return which goes to the called function.
>
> I'm remembering the 'active' instructions in a retpoline being 'push; ret'.
> Which is an RSB imbalance.

Looking at the code might help to remember correctly:

        call   1f
        speculation trap
1:      mov     %reg, %rsp
        ret

Thanks,

        tglx