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

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Date:   Sun, 17 Jul 2022 17:56:28 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Thomas Gleixner' <tglx@...utronix.de>,
        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

From: Thomas Gleixner
> Sent: 17 July 2022 16:07
> 
> On Sun, Jul 17 2022 at 09:45, David Laight wrote:
> > From: Thomas Gleixner
> >>
> >>  3) Utilize the retbleed return thunk mechanism by making the jump
> >>     target run-time configurable. Add the accounting counterpart and
> >>     stuff RSB on underflow in that alternate implementation.
> >
> > What happens to indirect calls?
> > The above would imply that they miss the function entry thunk, but
> > get the return one.
> > Won't this lead to mis-counting of the RSB?
> 
> That's accounted in the indirect call thunk. This mitigation requires
> retpolines enabled.

Thanks, that wasn't in the summary.

> > I also thought that retpolines would trash the return stack?
> 
> No. They prevent that the CPU misspeculates an indirect call due to a
> mistrained BTB.
> 
> > Using a single retpoline thunk would pretty much ensure that
> > they are never correctly predicted from the BTB, but it only
> > gives a single BTB entry that needs 'setting up' to get mis-
> > prediction.
> 
>   BTB != RSB

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.

...
> > I'm also sure I managed to infer from a document of instruction
> > timings and architectures that some x86 cpu actually used the BTB
> > for normal conditional jumps?
> 
> That's relevant to the problem at hand in which way?

The next problem :-)

	David

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