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From:   Sami Tolvanen <samitolvanen@google.com>
Date:   Mon, 18 Jul 2022 15:55:29 -0700
Message-ID: <CABCJKudvSv9bAOrDLHki5XPYNJK6=PS-x8v=E08es8w4LJpxBw@mail.gmail.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
To:     Joao Moreira <joao@overdrivepizza.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "Torvalds, Linus" <torvalds@linux-foundation.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 3:47 PM Joao Moreira <joao@overdrivepizza.com> wrote:
> Thus the hash will be 6 bytes before the function entry point. Then we
> can get the compiler to emit a padding area before the __cfi_\func
> snippet and, during boot, if the CPU needs the call depth tracking
> mitigation, we:
> - move the __cfi_func into the padding area
> - patch the call depth tracking snippet ahead of it (overwriting the old
> __cfi_\func:)
> - fix the cmpl offset in the caller
>
> func_whatever:
>         ...
>         cmpl $0x\hash, -FIXED_OFFSET(%rax)
>         je 1f
>         ud2
> 1:
>         call *%rax
>         ...

The problem with this is that the cmpl instruction contains the full
type hash, which means that any instruction that's FIXED_OFFSET from
the cmpl is a valid indirect call target as far as KCFI is concerned.
-6 was chosen specifically to make the ud2 the only possible target.

Sami