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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 18 Jul 2022 16:42:29 -0700
Message-ID: <CAHk-=wjEDJ4+xg0CWR7CaCKnO6Nhzn+vjJy7CjaVmf9R+g_3ag@mail.gmail.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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>,
        "Cooper, Andrew" <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>,
        "Moreira, Joao" <joao.moreira@intel.com>,
        "Nuzman, Joseph" <joseph.nuzman@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Gross, Jurgen" <jgross@suse.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Peter Collingbourne <pcc@google.com>
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On Mon, Jul 18, 2022 at 4:19 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> But that's an implementation detail, right? Whatever we put in between
> will still be a fixed offset, no? It's a different offset, but that's
> what patching can deal with.

No, what Sami is sayin that because the "cmpl" *inside* the function
that checks the hash value will have that same (valid) hash value
encoded as part of it, then you actually have *two* valid markers with
that hash value.

You have the "real" marker before the function.

But you also have the "false" marker that is part of the hash check
that is *inside* the function.

The "real marker + 6" points to the function head itself, and so is ok
as a target (normal operation).

The "false marker + 6" points to the "UD2", and so is *also* ok as a
target (bad guy trying to mis-use the false marker gets trapped by
UD2).

               Linus