linux-hardening - Re: [PATCH v2 50/63] tracing: Use memset_startat() to zero struct trace

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Date:   Wed, 18 Aug 2021 09:33:49 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        dri-devel@...ts.freedesktop.org, linux-staging@...ts.linux.dev,
        linux-block@...r.kernel.org, linux-kbuild@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 50/63] tracing: Use memset_startat() to zero struct
 trace_iterator

On Tue, 17 Aug 2021 23:05:20 -0700
Kees Cook <keescook@...omium.org> wrote:

> In preparation for FORTIFY_SOURCE performing compile-time and run-time
> field bounds checking for memset(), avoid intentionally writing across
> neighboring fields.
> 
> Use memset_startat() to avoid confusing memset() about writing beyond
> the target struct member.
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Signed-off-by: Kees Cook <keescook@...omium.org>
> ---
>  kernel/trace/trace.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
> index 13587e771567..9ff8c31975cd 100644
> --- a/kernel/trace/trace.c
> +++ b/kernel/trace/trace.c
> @@ -6691,9 +6691,7 @@ tracing_read_pipe(struct file *filp, char __user *ubuf,
>  		cnt = PAGE_SIZE - 1;
>  
>  	/* reset all but tr, trace, and overruns */
> -	memset(&iter->seq, 0,
> -	       sizeof(struct trace_iterator) -
> -	       offsetof(struct trace_iterator, seq));
> +	memset_startat(iter, 0, seq);

I can't find memset_startat() in mainline nor linux-next. I don't see it
in this thread either, but since this has 63 patches, I could have
easily missed it.

This change really should belong to a patch set that just introduces
memset_startat() (and perhaps memset_after()) and then updates all the
places that should use it. That way I can give it a proper review. In
other words, you should break this patch set up into smaller, more
digestible portions for the reviewers.

Thanks,

-- Steve



>  	cpumask_clear(iter->started);
>  	trace_seq_init(&iter->seq);
>  	iter->pos = -1;