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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 18 Jul 2022 17:03:49 -0700
Message-ID: <CAHk-=wiJNViWKCCrDPByGWmVVXuEKhRGykx4q8diXSxEqGfOMw@mail.gmail.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Sami Tolvanen <samitolvanen@google.com>,
        Joao Moreira <joao@overdrivepizza.com>,
        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 4:58 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, Jul 18, 2022 at 04:52:09PM -0700, Linus Torvalds wrote:
> > I also happen to believe that the kCFI code should have entirely
> > different targets for direct jumps and for indirect jumps, but that's
> > a separate issue. Maybe it already does that?
>
> kCFI is purely about indirect calls.

So it already only adds the pattern to things that have their address
taken, not all functions?

If so, that's simple enough to sort out: don't do any RSB stack
adjustment for those thunks AT ALL.

Because they should just then end up with a jump to the "real" target,
and that real target will do the RSB stack thing.

               Linus