linux-kernel - Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec

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Date:   Thu, 4 Jan 2018 04:44:24 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     "Williams, Dan J" <dan.j.williams@...el.com>
Cc:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "alan@...ux.intel.com" <alan@...ux.intel.com>,
        "Reshetova, Elena" <elena.reshetova@...el.com>,
        "mark.rutland@....com" <mark.rutland@....com>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "jikos@...nel.org" <jikos@...nel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
Subject: Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

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?