linux-kernel - Re: [PATCH v11 03/12] scs: add support for stack usage debugging

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Date:   Mon, 20 Apr 2020 18:17:42 +0100
From:   Will Deacon <will@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 03/12] scs: add support for stack usage debugging

On Thu, Apr 16, 2020 at 09:12:36AM -0700, Sami Tolvanen wrote:
> Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks. When enabled,
> also prints out the highest shadow stack usage per process.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> Reviewed-by: Kees Cook <keescook@...omium.org>
> ---
>  kernel/scs.c | 39 +++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/kernel/scs.c b/kernel/scs.c
> index 5245e992c692..ad74d13f2c0f 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -184,6 +184,44 @@ int scs_prepare(struct task_struct *tsk, int node)
>  	return 0;
>  }
>  
> +#ifdef CONFIG_DEBUG_STACK_USAGE
> +static inline unsigned long scs_used(struct task_struct *tsk)
> +{
> +	unsigned long *p = __scs_base(tsk);
> +	unsigned long *end = scs_magic(p);
> +	unsigned long s = (unsigned long)p;
> +
> +	while (p < end && READ_ONCE_NOCHECK(*p))
> +		p++;

I think the expectation is that the caller has already checked that the
stack is not corrupted, so I'd probably throw a couple of underscores
in front of the function name, along with a comment.

Also, is tsk ever != current?

> +
> +	return (unsigned long)p - s;
> +}
> +
> +static void scs_check_usage(struct task_struct *tsk)
> +{
> +	static DEFINE_SPINLOCK(lock);
> +	static unsigned long highest;
> +	unsigned long used = scs_used(tsk);
> +
> +	if (used <= highest)
> +		return;
> +
> +	spin_lock(&lock);
> +
> +	if (used > highest) {
> +		pr_info("%s (%d): highest shadow stack usage: %lu bytes\n",
> +			tsk->comm, task_pid_nr(tsk), used);
> +		highest = used;
> +	}
> +
> +	spin_unlock(&lock);

Do you really need this lock? I'd have thought you could cmpxchg()
highest instead.

Will