linux-kernel - Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

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Date:   Thu, 24 Feb 2022 16:58:41 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        jpoimboe@...hat.com, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, ndesaulniers@...gle.com,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 15/39] x86/ibt,kprobes: Fix more +0 assumptions

On Thu, Feb 24, 2022 at 03:51:53PM +0100, Peter Zijlstra wrote:
> With IBT on, sym+0 is no longer the __fentry__ site.
> 
> NOTE: the architecture has a special case and *does* allow placing an
> INT3 breakpoint over ENDBR in which case #BP has precedence over #CP
> and as such we don't need to disallow probing these instructions.
> 
> NOTE: irrespective of the above; there is a complication in that
> direct branches to functions are rewritten to not execute ENDBR, so
> any breakpoint thereon might miss lots of actual function executions.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  arch/x86/kernel/kprobes/core.c |   11 +++++++++++
>  kernel/kprobes.c               |   15 ++++++++++++---
>  2 files changed, 23 insertions(+), 3 deletions(-)
> 
> --- a/arch/x86/kernel/kprobes/core.c
> +++ b/arch/x86/kernel/kprobes/core.c
> @@ -1156,3 +1162,8 @@ int arch_trampoline_kprobe(struct kprobe
>  {
>  	return 0;
>  }
> +
> +bool arch_kprobe_on_func_entry(unsigned long offset)
> +{
> +	return offset <= 4*HAS_KERNEL_IBT;
> +}

Let's avoid magic (though obvious right now) literal values. Can the "4"
be changed to a new ENBR_INSTR_SIZE macro or something?

-- 
Kees Cook