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Date:   Wed, 20 Jul 2022 12:36:38 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.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
Message-ID: <202207201219.8EA905372@keescook>
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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/

-- 
Kees Cook