Date: Thu, 4 Jan 2018 04:44:24 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: "Williams, Dan J" <dan.j.williams@intel.com>
Cc: "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "peterz@infradead.org" <peterz@infradead.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "alan@linux.intel.com" <alan@linux.intel.com>,
        "Reshetova, Elena" <elena.reshetova@intel.com>,
        "mark.rutland@arm.com" <mark.rutland@arm.com>,
        "gnomes@lxorguk.ukuu.org.uk" <gnomes@lxorguk.ukuu.org.uk>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "jikos@kernel.org" <jikos@kernel.org>,
        "linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Subject: Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier
Message-ID: <20180104044424.GC21978@ZenIV.linux.org.uk>
References: <20180103223827.39601-1-mark.rutland@arm.com>
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 <20180104010754.22ca6a74@alans-desktop>
 <alpine.LRH.2.00.1801040221070.27010@gjva.wvxbf.pm>
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On Thu, Jan 04, 2018 at 03:10:51AM +0000, Williams, Dan J wrote:

> diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
> index 1c65817673db..dbc12007da51 100644
> --- a/include/linux/fdtable.h
> +++ b/include/linux/fdtable.h
> @@ -82,8 +82,10 @@ static inline struct file *__fcheck_files(struct files_struct *files, unsigned i
>  {
>  	struct fdtable *fdt = rcu_dereference_raw(files->fdt);
>  
> -	if (fd < fdt->max_fds)
> +	if (fd < fdt->max_fds) {
> +		osb();
>  		return rcu_dereference_raw(fdt->fd[fd]);
> +	}
>  	return NULL;
>  }

... and the point of that would be?  Possibly revealing the value of files->fdt?
Why would that be a threat, assuming you manage to extract the information in
question in the first place?