linux-kernel - Re: [PATCH] posix-timers: Use array safe helper when fetching notification symbolic name

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Date:   Fri, 9 Nov 2018 10:18:25 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Cyrill Gorcunov <gorcunov@...il.com>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrey Vagin <avagin@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] posix-timers: Use array safe helper when fetching
 notification symbolic name

On Thu, 1 Nov 2018, Cyrill Gorcunov wrote:

> When showing timer's notify symbolic name make sure we never fetch a value
> sitting outside of the names array. Though the former issue displaying
> timer->it_sigev_notify has been fixed by Thomas in commit cef31d9af9082434,
> better to make sure we won't hit it again in furher modifications.
> 
> Cc: Andrey Vagin <avagin@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Cyrill Gorcunov <gorcunov@...il.com>
> ---
>  fs/proc/base.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> Index: linux-ml.git/fs/proc/base.c
> ===================================================================
> --- linux-ml.git.orig/fs/proc/base.c
> +++ linux-ml.git/fs/proc/base.c
> @@ -2287,7 +2287,7 @@ static int show_timer(struct seq_file *m
>  {
>  	struct k_itimer *timer;
>  	struct timers_private *tp = m->private;
> -	int notify;
> +	int notify, nidx;
>  	static const char * const nstr[] = {
>  		[SIGEV_SIGNAL] = "signal",
>  		[SIGEV_NONE] = "none",
> @@ -2296,13 +2296,13 @@ static int show_timer(struct seq_file *m
>  
>  	timer = list_entry((struct list_head *)v, struct k_itimer, list);
>  	notify = timer->it_sigev_notify;
> +	nidx = array_index_nospec(notify & ~SIGEV_THREAD_ID, ARRAY_SIZE(nstr));

I completely understand your intention, but this is misleading. The above
is really not a speculation gadget.

I'd rather do an open coded check here and fail the thing instead of
printing wrong information:

    	   nidx = timer->it_sigev_notify & ~SIGEV_THREAD_ID;
	   if (nidx >= ARRAY_SIZE(nstr))
	   	return -EINVAL;

Thanks,

	tglx