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Date:   Wed, 20 Jul 2022 23:36:08 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kees Cook <keescook@chromium.org>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Josh Poimboeuf <jpoimboe@kernel.org>,
        Andrew Cooper <Andrew.Cooper3@citrix.com>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
        Johannes Wikner <kwikner@ethz.ch>,
        Alyssa Milburn <alyssa.milburn@linux.intel.com>,
        Jann Horn <jannh@google.com>, "H.J. Lu" <hjl.tools@gmail.com>,
        Joao Moreira <joao.moreira@intel.com>,
        Joseph Nuzman <joseph.nuzman@intel.com>,
        Juergen Gross <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
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On Wed, Jul 20, 2022 at 12:36:38PM -0700, Kees Cook wrote:
> On Wed, Jul 20, 2022 at 11:07:26AM -0700, Linus Torvalds wrote:
> > On Wed, Jul 20, 2022 at 10:50 AM Steven Rostedt <rostedt@goodmis.org> wrote:
> > >
> > > [    2.464117] missing return thunk: lkdtm_rodata_do_nothing+0x0/0x8-lkdtm_rodata_do_nothing+0x5/0x8: e9 00 00 00 00
> > 
> > Well, that looks like a "jmp" instruction that has never been relocated.
> 
> Peter, Josh, and I drilled down into this recently[1] and discussed
> some solutions[2].
> 
> This test is doing what's expected: it needed an arch-agnostic way to do
> a "return", and when the way to do that changed, it also changed (which
> would normally be good, but in this case broke it). It's been happily
> being used as part of the per-section architectural behavior testing[3]
> of execution-vs-expected-memory-permissions for quite a long while now.
> 
> I'd rather not remove it (or do it dynamically) since the point is to
> test what has been generated by the toolchain/build process and stuffed
> into the .rodata section. i.e. making sure gadgets there can't be
> executed, that the boot-time section permission-setting works correctly,
> etc. Before the retbleed mitigation, this test worked for all
> architectures; I'd hate to regress it. :(
> 
> -Kees
> 
> [1] https://lore.kernel.org/lkml/Ys66hwtFcGbYmoiZ@hirez.programming.kicks-ass.net/
> [2] https://lore.kernel.org/lkml/20220713213133.455599-1-keescook@chromium.org/
> [3] e.g. https://linux.kernelci.org/test/plan/id/62d61ee8ef31e0f0faa39bff/

Josh posted this:

  https://lkml.kernel.org/r/8ec0039712f252693049c70ed3891d39a2357112.1658155446.git.jpoimboe@kernel.org

which I picked up today; barring robot fail I'll push it to x86/urgent
tomorrow.