linux-kernel - Re: [PATCH] objtool,x86: Verify poke_int3

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Message-ID: <20200727112144.GA55660@gmail.com>
Date:   Mon, 27 Jul 2020 13:21:44 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     peterz@...radead.org
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org, mbenes@...e.cz
Subject: Re: [PATCH] objtool,x86: Verify poke_int3_handler() is self contained


* peterz@...radead.org <peterz@...radead.org> wrote:

> 
> Abuse the SMAP rules to ensure poke_int3_handler() doesn't call out to
> anything.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  tools/objtool/check.c |    8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -551,6 +551,14 @@ static const char *uaccess_safe_builtin[
>  	"__memcpy_mcsafe",
>  	"mcsafe_handle_tail",
>  	"ftrace_likely_update", /* CONFIG_TRACE_BRANCH_PROFILING */
> +	/*
> +	 * Abuse alert!
> +	 *
> +	 * poke_int3_handler() is not in fact related to uaccess at all, we
> +	 * abuse the uaccess rules to ensure poke_int3_handler() is self
> +	 * contained and doesn't CALL out to other code.
> +	 */
> +	"poke_int3_handler",

So ->uaccess_safe makes sure that we don't CALL into non-uaccess-safe 
functions, but it still allows CALLs into *other* uaccess-safe 
functions, right?

So unless I missed something in the logic, the comment should say 
something like "doesn't CALL out to other non-uaccess safe code" or 
so? Which is, arguably, like 99% of all functions - but still, a whole 
bunch are allowed, such as low level instrumentation and other utility 
functions.

Thanks,

	Ingo