linux-kernel - Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

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Date:   Mon, 18 Jul 2022 15:55:29 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Joao Moreira <joao@...rdrivepizza.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Torvalds, Linus" <torvalds@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        "Moreira, Joao" <joao.moreira@...el.com>,
        "Nuzman, Joseph" <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Gross, Jurgen" <jgross@...e.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Peter Collingbourne <pcc@...gle.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

On Mon, Jul 18, 2022 at 3:47 PM Joao Moreira <joao@...rdrivepizza.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